Gaiden Game

A game which takes place in or refers to another video game, but isn't really a sequel. It can be a simple side story, a Perspective Flip, or a chance to give a liked character background they didn't get in the original game. The major stipulation is it is not usually required to canonically fit into the "main" game or require having played it to enjoy.

Gaiden Games are sometimes titled from the direct translation of the Japanese word gaiden, meaning "another story". Frequently, these games are released on portables or "weaker" systems, but modified appropriately. They are often lower budget and can be seen as cash-ins, but can be interesting if they choose a different viewpoint, poke fun at the original, or are simply more innovative than a large-budget game might be allowed to be.

Not to be confused with Ninja Gaiden, which has a troubled approach with its gaiden status. See the respective entry below.

Also of note is that having Gaiden in its title doesn't necesseraly means the game pertains to this trope.


 * There's a couple in the Sonic the Hedgehog series. Tails Adventure springs to mind. In the Japanese manual its said to take place before Tails' fateful meeting with his iconic partner in crime fighting. In the Western version it's treated as a Busman's Holiday but in both, its still Tails's day in the limelight.
 * As do the Sonic Storybook Series and Sonic Battle.
 * Shin Megami Tensei: if... started off originally as a gaiden game of sorts in the Shin Megami Tensei universe, taking place just before Shin Megami Tensei I. It later became canon when the protaganist appeared in Persona and Persona 2, and started the Intercontinuity Crossover that occurs throughout the Megaten franchise.
 * It's not quite clear how Shin Megami Tensei: Strange Journey fits in - which is really odd since it's one of the four main series games. The other three are all clearly connected (2 is 1's sequel, 3 is all the stuff happening that 1 & 2 are part of, and 4 is...um...)
 * There are more spinoff games then there are main series games. Hell, there are more games in the Persona series than in the main series.
 * The first three Tomb Raider games were eventually re-released as the "Tomb Raider Gold" series, and each game got its own Gaiden Game. TR1 had Unfinished Business, TR2 had Golden Mask and TR3 had Lost Artifact.
 * The downloadable game Lara Croft and the Guardian of Light seems to also be this, not taking place in the continuities of the original Core Design series or the Crystal Dynamics-developed games.
 * Each Donkey Kong Country game had a Game Boy follow-up in the "Donkey Kong Land" series. The first was a completely separate adventure, the second was basically a port of Donkey Kong Country 2 and the third was a basic collection of very generic levels in the style of Donkey Kong Country 3.
 * Sailor Moon: Another Story was not so much a franchise distancer as a nod that it is not canonical to the Sailor Moon mythos in very Broad Strokes.
 * Much the same is true of Dragon Ball Z Gaiden: Plan to Eliminate the Saiyans. Its story has no bearing on the manga or anime, though Toei did produce a companion OVA.
 * As well as Zoids Legacy, which is like a Mega Crossover Fanfic of all the series continuums in video game form.
 * Final Fantasy X-2 orginally was informally referred to as a Gaiden Game before being treated as a direct sequel. Largely existing as an exercise in producing a sequel and light-hearted enough to occasionally take the piss out of its premise and characters, it was mainly dismissed in the West for its playful cuteness overt sexism (even for a Japanese game).
 * Even before X-2, the developers weren't sure if Final Fantasy IX would be considered part of the main franchise due to how much is deviated from Final Fantasy VII and Final Fantasy VIII; it was less than a year before release that Square officially called it IX.
 * The recent spinoff games to the VII universe could be considered Gaiden Games, including the PlayStation 2 sequel Dirge of Cerberus, and two prequels, Before Crisis for mobile phones and Crisis Core on the PSP. Fans are divided how much material has been stapled on as a cash grab and how much was simply cut for time.
 * Also in the PSP library, we've got Daxter (taking place before the main events of Jak II Renegade), God of War: Chains of Olympus (a prequel to the original game), Grand Theft Auto Liberty City Stories and Grand Theft Auto Vice City Stories (prequels to Grand Theft Auto III and Grand Theft Auto Vice City, respectively), Ratchet and Clank Size Matters and Secret Agent Clank, and Metal Gear Solid Portable Ops (taking place between Metal Gear Solid 3: Snake Eater and Metal Gear Solid Peace Walker).
 * Portable Ops is unique in that while you don't have to play it to understand the story of Metal Gear, its makes it easer to understand the story as the ending reveals how The Patriots where created, and fills in how the Philosophers became the Patriots.
 * The Legend of Zelda:
 * The Legend of Zelda Majoras Mask was essentially one giant sidequest. In fact, the game's working title was Zelda Gaiden. It was supposed to be an add-on of sorts to The Legend of Zelda Ocarina of Time using the 64DD peripheral.
 * In general, Nintendo seems to like to make almost every other Zelda game a Gaiden Game. The Legend of Zelda Links Awakening and The Legend of Zelda Phantom Hourglass are also Gaiden Games. The Four Swords trilogy was supposed to be made up of these, and then Four Swords Adventures shoehorned Ganon into the plot and connected it to the main series.
 * The original Ninja Gaiden trilogy for the NES, along with the arcade game released alongside the first NES installment, weren't actually side-stories to anything. In Japan, the series was originally known as Ninja Ryūkenden (Ninja Dragon Sword Legend). The use of "gaiden" in the English version is an example of Gratuitous Japanese, since the developers were not sure how to localize the Japanese title ("Ninja Dragon" was considered one point, but Data East beat them to it with their beat-'em-up Bad Dudes vs. Dragon Ninja, and a literal translation was considered to be too long). With that cleared up, Ninja Gaiden Dragon Sword could be considered one to the Xbox series.
 * Galaxy Angel EX is a non-canonical glorified giant minigame.
 * The aptly named Fire Emblem Gaiden is a side-story to the original Fire Emblem, while Fire Emblem: Thracia 776 is one to Genealogy of Holy War. The two titles are generally considered full instalments in the Fire Emblem series, being the second and fifth respectively, though there are some portions of the Japanese fanbase who don't consider Gaiden to be a full FE game. Also present are the Satellaview instalments in the series, which are briefer games focusing on small groups of the cast of the first/third game doing things in the intervening time period between Dolhr's victory and the start of the first/third games; they were later remade and included as a bonus in New Mystery of the Emblem.
 * It could be argued that Xenosaga 2 and 3 were Gaiden Games, not to Xenosaga, but rather to Xenogears. There's a lot of legal difficulties in the connections between those, so just Wikipedia Xenosaga to learn more about the connections (and lack thereof).
 * Castlevania: When Koji Igarashi took over as producer of the Castlevania games (starting with 2002's Harmony of Dissonance), he declared that the Nintendo 64 games ( Castlevania64 and Castlevania: Legacy of Darkness), and Circle of the Moon for the Game Boy Advance were side-stories to the main Castlevania storyline. The Game Boy game Castlevania: Legends on the other hand, is no longer part of the canon.
 * In Japan, Castlevania: Legacy of Darkness was titled Akumajō Dracula Mokushiroku Gaiden: Legend of Cornell, making it a Gaiden Game to the earlier N64 title, whose Japanese title was Akumajō Dracula Mokushiroku.
 * Dawn Of Sorrow. Despite continuing the story directly from Aria of Sorrow, it has next to nothing to do with either that title or the series' overall Myth Arc. (Unless the Julius Mode ending, where proves to be canon.) It doesn't even take place in the titular castle.
 * The Boku Dracula-kun games for the Famicom and Game Boy (the latter brought over to the U.S. as Kid Dracula) were never intended to be canon, though the Big Bad, Galamoth, would later appear in the regular series.
 * Phantasy Star Gaiden was originally intended to fill in some side events to the series to act as the lead-in to an earlier concept for Phantasy Star 4. As that game ended up using a different storyline in the final version, and there hasn't been a single game released in that continuity since, said Gaiden is now meaningless to the overall continuity.
 * Many gaidens are found in the Mega Man franchise—in fact, each series seems to get at least one. Typical examples are Mega Man and Bass, Mega Man X Command Mission, Mega Man Legends: The Misadventures of Tron Bonne, and Mega Man Battle Network: Battle Chip Challenge.
 * The first two Shining Force Gaiden games (Game Gear) were eventually bundled under the name Shining Force CD (Sega CD). And just to be confusing, Shining Force Gaiden III: Final Conflict is unrelated to the previous two Gaidens (aside from being on Game Gear) and is instead a bridge taking place between the first two 'proper' Shining Force games.
 * The GBA title Kingdom Hearts: Chain of Memories is frequently considered a Gaiden Game to the main Kingdom Hearts series, but Word of God has said otherwise and not playing it makes Kingdom Hearts II make even less sense.
 * The same can be said about Birth By Sleep, coded, and 358/2 Days. One can understand the first two/three installments just fine without the latter three games, but they all connect the dots between "main games", are just as canon and relevant as the main games, and future games require passing background knowledge of them in order to fully enjoy the story. Birth By Sleep, for example, is actually acknowledged as "Kingdom Hearts 0" as part of the main storyline.
 * The GBA remake of Final Fantasy II contained a short quest after beating the game, detailing what happened to all the dead party members after they died.
 * Portal is almost an inverted example; everything (bar a couple of throwaway references to Black Mesa) was original, and the connections to the Half Life story were made in the main Half Life series.
 * Blue Shift, Opposing Force and Decay could probably also be considered Gaiden Games. Especially Opposing Force, which while set in the same time-frame as the original, introduced new aliens (Race X) and characters (Cpl. Adrian Shepard, Security guard Otis Laurey) that have not been considered "canonical" by Valve.
 * Super Robot Wars has three Gaiden games with each of them a part of the three major continuties ("Classic", Alpha, and Original Generation). The first Gaiden game Super Robot Wars Gaiden: Lord of Elementals told of the origins of the Masou Kishin characters, a group of Banpresto-created originals not seen anywhere else. Super Robot Wars Alpha Gaiden focused on Time Travel and wasn't necessary for players to enjoy the previous Alpha game (most likely because Banpresto wanted an excuse to show off obscure mecha series, since it was full of them). Super Robot Wars Original Generation Gaiden fits this trope because it was shorter than the average SRW, including several extras such as a battle viewer and a card game. It's also downplayed, though, since all three are essentially sequels that happen to have the word "gaiden" in their name. Super Robot Wars Alpha depends on the player having some foreknowledge of the events set in Super Robot Wars EX or Super Robot Wars Gaiden for background on the Masou Kishin characters, otherwise one can get too confused at the references they make to Alpha's back story. Alpha Gaiden is heavily referenced in the proper sequel Alpha 2, where the Dinosaur Empire is defeated for the third time, and the finale Alpha 3 assumes the player knows of Sanger Zonvolt's role at the Earth Cradle, despite the fact it was supposed to be highly secretive. Hell, the fact the Titans are more or less liberally screwed and Char Anzable's disillusionment with humanity DEPENDS on the events of Alpha Gaiden. In short, Banpresto's definition of "gaiden" means a game that provides story details bridging the gap and answering the Epileptic Trees present in the other games in continuity. In fact, there's very little an "Original Generation 3" couldn't reference the events of Original Generation Gaiden, considering both the effects on existing characters and all the EarlyBirdCameoes present in that game.
 * A better example is Super Robot Wars OG Saga: Endless Frontier. While a spin-off, the back-story establishes the events in Original Generation continuity ultimately influenced the entirety of Endless Frontier. Its upcoming sequel Endless Frontier EXCEED even manages to rope in characters from the main Original Generation games.
 * Amusingly, a remake of the original Super Robot Wars Gaiden has been announced, only it now carries the "OG Saga" subtitle instead. Thus, the name "Gaiden" has become reserved for half-sequels while "OG Saga" is given to the actual Gaiden Games.
 * Chrono Cross is somewhat of a Gaiden Game for Chrono Trigger, being set 10 years after the "present" time in the latter and retaining only a handful of characters, all of whom show up in three scenes or fewer. What really makes it gaiden, though, is the fact that, in the end, the entire point of the story is to resolve a hanging plot thread from its predecessor (see Urban Legend of Zelda).
 * Radical Dreamers was a Japan-only text adventure Gaiden Game to Chrono Trigger released on the SNES' Satellaview addon. It was later overhauled, greatly expanded, turned into a proper RPG... and became Chrono Cross.
 * The Worlds of Ultima series were Gaiden Game's taking the fantasy-based Ultima VI engine (and main character) to other settings, such as Mars. Ultima Underworld was also a Gaiden Game, being a side story set in the main Ultima world but with a completely different interface and gameplay. Ultima Underworld II was not a Gaiden Game, because its plot directly bridged Ultima VII and Ultima VII Part 2 (in fact, in the latter the PC starts with a quest item obtained in UU2 with no in-game explanation of where it came from).
 * The two console games from the mid 90s, Ultima: Runes of Virtue and Ultima: ROV 2, would certainly count as well. Both games are set in the usual Ultima game world, and feature characters and towns familiar from the parent series. But both games are more like action games than RPGs, and neither one is part of the official Ultima chronology. They are, like Underworld, a separate mini-series of their own.
 * There are two Japan-only games in the Suikoden series called, quite simply, Suikogaiden volumes 1 and 2. These games are basically side-stories featuring a previously-unknown character from Harmonia named Nash Latkje (who would later appear as a Star of Destiny in Suikoden III). The two games take place around the time of Suikoden 2, the first starting before and during SII, and the second taking place shortly after the end of SII. In both games, Nash interacts with various characters from Suikoden II, giving more perspective on many of the less known-about characters.
 * Also Suikoden Tactics/Rhapsodia, set just after Suikoden IV.
 * Melty Blood, the rather popular 2D Fighter Gaiden Game to Tsukihime, which follows a plotline of many that didn't quite make it into the actual visual novel.
 * Trilby: The Art of Theft is Yahtzee's Gaiden Game for the Chzo Mythos series, featuring as it does one of the main characters years before the series proper starts, and an Unexpected Gameplay Change to stealth platforming.
 * While not directly linked to another game, Indiana Jones & the Fate of Atlantis could be considered a Gaiden Game to the Indiana Jones series of movies.
 * After the original Guilty Gear, X and X2 are sidestories like the drama cds. The main plot is now in Guilty Gear 2: Overture. (Don't worry, Word of God still says it's canon...mostly...especially with the release of Accent Core plus...which seems to tell the story on how they got to Overture.)
 * Phoenix Wright: Ace Attorney's bonus case, only present on the DS version (The original Japan-only GBA version ends at the fourth case), features a case where only 5 characters (Phoenix, Edgeworth, Gumshoe, the Judge and the Bellboy) from the rest of the series appear, the rest being completely new. This is due to the case taking place between the first and second games, and the writers couldn't mess with the continuity already set by the sequels which had already been released in Japan. The plot and characters feel perfectly like a sidestory.
 * The fifth case has been worked into the Canon with Apollo Justice featuring Ema as the game's Gumshoe.
 * Gyakuten Kenji (or Miles Edgeworth: Ace Attorney Investigations), a Gaiden Game where you play as Miles Edgeworth, Nick's rival. It follows the same general formula except that Edgeworth is actully on the map as a sprite and walks around rather then looking at a static image. There is no court segments (Unless the case taking place in a court house counts), but witnesses are still cross examined in much the same manner as the main series. It now has its own sequel, becoming a Gaiden Series.
 * Resident Evil Gaiden was the actual name of the game. Its ending has since been decanonized without doubt.
 * The light gun Spin Offs could also count, though they have slightly more credible ties to the Canon. Some even seem to be retcon vehicles.
 * RE 3: Nemesis may be considered a gaiden to the second game.
 * The Dead or Alive XTreme games are (even more) fanservicey gaiden games of Dead or Alive.
 * The Elder Scrolls had an action-RPG, An Elder Scrolls Legend: Battlespire, and an action-adventure, The Elder Scrolls Adventures: Redguard (another Elder Scrolls Adventures, The Eye of Argonia, was planned but never made, though the Eye itself is mentioned in the main games). (Those who don't know this often erroneously assume that it's a reference to The Eye of Argon.) There's also the Elder Scrolls Travels side-series, consisting of Dawnstar, Stormhold, and the totally not canon Shadowkey, for the ill-fated N-Gage system.
 * Gradius Gaiden, the only Gradius title that allows the player to rearrange the power meter, and the second non-Parodius game to have multiple selectable ships (the MSX title Nemesis 3 being the first). And for that matter, the MSX version of Salamander, and MSX exclusives Nemesis 2 and 3. The MSX Salamander plays more like a Gradius title, and has several new features such as a powerup that temporarily stops the screen scrolling. Nemesis 2 is an original title with the ability to fly into about-to-be-destroyed boss ships and obtain new powerups such as an upward-firing laser, at the cost of having a longer power meter. Nemesis 3 is a retelling of the more mainstram Gradius II: Gofer no Yabou with Nemesis 2-style gameplay. Also, the Salamander series is a gaiden series to Gradius.
 * Warhammer Fantasy Battle and Warhammer 40000 each have a number of spinoff tabletop games in their universes. Warhammer Fantasy's games include Warhammer Fantasy Roleplay, the gang-based Mordheim, American football parody Blood Bowl (even referring to the mystical god 'Nuffle') and massive-battle Warmaster. 40k has the large-scale "narrative wargame" Inquisitor, space combat Battlefleet Gothic, small-scale Epic, all-Ork Gorkamorka, air-battle Aeronautica Imperialis, gang-based Necromunda and recently the role-playing game Dark Heresy.
 * Dawn of War was originally supposed to be this, creating an all-new Space Marine chapter, the Blood Ravens, to avoid stepping on continuity's toes. The fans liked them so much Games Wrokshop went ahead and canonized them, the games, and the events therein.
 * Soul Calibur 2's Weapon Master Mode appears to be a gaiden storyline: Weapon Master Mode takes place...somewhere other than Europe and Asia. Also, none of the Soul Calibur characters appear in Weapon Master Mode; the characters in Weapon Master Mode use the Soul Calibur fighters as "avatars", i.e. you're not actually fighting Mitsurugi, you're fighting some guy named Edgar.  Boy is this complicated.
 * Weapon Master Mode is definitively a gaiden, likely not related to the main plot (thought there are a few hints of it being a tale from the distant past, like . Oh, and.
 * In Soul Calibur 3, it was a Euro-Asian conflict, by chance, happened to be in the areas where the characters looking for Soul Edge.
 * Chronicles of the Sword is an Alternate Universe, set on a ficticious continent with ficticious countries, and starring the Soul cast as mere cameos with no storyline relevance. It's not part of the main canon.
 * Unreal Tournament is a Gaiden Series to the Unreal series, taking place within the Unreal universe but having little to do with the Skaarj invasion.
 * And the Unreal Championship games for consoles are a spinoff from Unreal Tournament, creating a Gaiden Game of a Gaiden Game.
 * Because Everything's Better with Penguins, Turn-Based Strategy series Disgaea has a platformer spin-off for the PSP called Prinny: Can I Really Be the Hero? starring everyone's favorite explosive waterfowl squad.
 * The DJ MAX series has primarily been a Beatmania-like game, but the newly-released arcade game DJ MAX Technika is a much different game, with touchscreen-based gameplay combining elements of Osu! Tatakae! Ouendan/Elite Beat Agents and Lumines. Due to its similarities to the former and its harsh Life Meter, it's a very Nintendo Hard game; you can easily fail a song in the first 10 seconds.
 * Higurashi Daybreak, a doujin game for Higurashi no Naku Koro ni that's literally become a canon side story.
 * There's also Jan, in which the characters can (depending on the mode) go crazy and kill each other just like usual, and they're dueling with... mah-jong?
 * Similarly, Umineko no Naku Koro ni will soon have its own Gaiden Game in the form of Umineko No Naku Koro Ni - Tsubasa, and will contain all the side stories released beforehand.
 * Mortal Kombat's action games: MK Mythologies: Sub-Zero, MK: Special Forces (starring Jax), and the more recent MK: Shaolin Monks with Liu Kang and Kung Lao.
 * Also the Konquest Mode from MK: Deception, which starts 50 years before the main game's story and ends in the beginning of Deception, and shows the story of one of the characters' exploits while.
 * Destroy All Humans!! Big Willy Unleashed is a Gaiden Game in the Destroy All Humans! series.
 * The Deus Ex Game Mod Zodiac has JC Denton's brother Paul Denton investigate a separate conspiracy.
 * Jet Set Radio Future is this to Jet Set Radio. It doesn't continue off of or add to the continuity of the original game. If anything, it's set in an alternate timeline.
 * The Enhanced Edition of The Witcher has two additional stories named 'Side Effects' and 'The Price of Neutrality', which are completely unrelated to the main game, but feature locations and characters known from there.
 * Barkley, Shut Up and Jam: Gaiden: A spinoff RPG from the original sports game Barkley: Shut up and Jam! There's also that Space Jam is also part of the game's canon.
 * R-Type Leo has gameplay significant from a "real" R-Type game. Instead of a Force Pod, you had two smaller pods that provided additional firepower and had a homing charge attack, and the plot takes place before any other games in the series. The obscure Armed Police Unit Gallop is also a Gaiden Game.
 * Clannad: Tomoyo After: It's a Wonderful Life.
 * Halo Wars is a real-time-strategy spin-off/prequel to the main series. Halo has another gaiden game of sorts in its expansion pack Halo 3: ODST. Despite that 3 in the title, it actually takes place during/just after Halo 2, and involves some new characters.
 * Devil May Cry 2 is a two-disc set. The second disk, which you may think will extend the story, doesn't. It fits this trope by giving you a gaiden game in form of Lucia, letting you play as her for the parts of the story where she wasn't interacting with Dante. It makes little enough sense what she's doing that it could easily be considered a wholly different game played in the DMC format.
 * The first Street Fighter EX originally had the working title Street Fighter Gaiden and the plot of the EX series (what little it has) is considered a side-story to Street Fighter II.
 * Despite being a small series (In terms of the number of entries, at least), Punch-Out!! has one of these: Arm Wrestling. It used the same two-screen arcade cabinet style of the original 2 games, its art style was similar to the Punch-Out!! series at the time, the main character resembled the arcade version of Little Mac (who had green hair), and arm wrestler Mask X, once his titular mask is removed, is revealed to be.
 * Metal Gear: Ghost Babel and the two Metal Gear Acid games are officially recognize as "Gaiden" games in the Metal Gear series (see here), even though they don't really fit in the series' canon in any way.
 * Christmas Ni GHTS is this for the Ni GHTS series. However, it was added onto the Updated Rerelease only in Japan.
 * The numbering of the four * .5 games (Immaterial and Missing Power, Shoot the Bullet, Scarlet Weather Rhapsody and Double Spoiler) would imply that they're all gaiden games to the main Touhou Project series (Touhou 12.3, Hisoutensoku, is an Expansion Pack to SWR), especially since none of them use the same gameplay system (IaMP and SWR are both 2D Fighting Games, and StB and DS are Boss Rushes where you take pictures instead of fighting back). However, of the four, StB and DS are the only ones without an actual plot—whereas the events and new character introduced in IaMP are acknowledged in the canon books Perfect Memento in Strict Sense, Bohemian Archive in Japanese Red, and Silent Sinner in Blue (SWR was made after those books came out).
 * IaMP boss character Suika Ibuki appears in Subterranean Animism, and the game actually elaborates on some of IaMP's plot (that is, where the oni all went). Iku Nagae and Tenshi Hinanai from SWR are both in The Grimoire of Marisa. The newest Gaiden Game, Touhou 12.8 Great Fairy Wars, is a direct continuation to a chapter of a Touhou manga, Strange and Bright Nature Deity. Basically, Touhou is undergoing Continuity Creep.
 * Pokémon has had a bunch of side games. An incomplete list: the First-Person Snapshooter game Pokémon Snap; the Puzzle Game Pokemon Trozei!; and the Pokémon Mystery Dungeon and Pokémon Ranger series. Likewise, the main series games are all Gaiden Games of each other, with references and allusions but no actual interaction. Mystery Dungeon & Ranger also have references and allusions but no interactions to their own series's, so gaiden games that are gaiden games of each other...
 * Warriors Orochi is a gaiden crossover between Samurai Warriors and Dynasty Warriors
 * Yggdra Unison is a gaiden game to Yggdra Union; it allows the player to command any army, make any alliances he or she wants, and aim for world domination in a Wide Open Sandbox style of play. The game is considerably more lighthearted than its canon counterpart, and concentrates on character development and interaction over story.
 * Breath of Fire IV has had a minor constellation of Gaiden Game treatments—at least two of them being released (along with a Comic Book Adaptation of IV) fully seven years after the original release. These last two, Breath of Fire IV - The Sword of Flame & the Magic of Wind and Breath of Fire IV: Faeries Light Key, are two separate side-stories of IV. There's also a spinoff of the fishing minigame from IV as well as a "Great Dalmuti"/"Millionaire" game featuring characters from IV. Unfortunately, due to the platform these were released on (Qualcomm's BREW OS, which is only common in Japan) these are likely to remain No Export for You permanently—much to the vexation of the English-speaking IV fandom.
 * The Mass Effect series has had multiple examples of this:
 * Mass Effect: Galaxies (for the iPod Touch/iPhone) focuses on Jacob Taylor and Miranda Lawson between the events of the first and second game. Completing Galaxies unlocks more dialogue in Mass Effect 2.
 * The iOS game Mass Effect: Infiltrator runs concurrently with the events of Mass Effect 3, and follows an ex-Cerberus operative who works to free a number of captive civilians from Cerberus' laboratories. The game has similar mechanics to the main game, and completing it allows the player to export a War Asset and a weapon over to 3.
 * Mass Effect: Datapad is another iOS game integrated with the third installment, and includes a galactic Codex, the ability to receive personal messages from squadmates and various characters in the universe, and a strategy minigame that allows you to increase your Galactic Readiness in the main game.
 * Metal Slug had a canon gaiden game on the Game Boy Advance, detailing a new training facility that was overrun by Morden's forces. Best of all, two of the trainees, playable characters Walter and Tyra, single-handedly take it all back.
 * The Front Mission spinoff Gun Hazard, which is a sidescrolling platformer-shooter compared to the isometric/3d turn-based strategy of the main games.
 * Kud Wafter.
 * Most of Medal of Honor: Frontline, except for the D-Day prologue, is set in between the third and fourth missions of the original game. Allied Assault also has a few continuity nods to that.
 * Time Crisis: Project Titan, Crisis Zone, and Razing Storm.
 * To clarify, Project Titan was a PSX-only sequel starring Richard Miller. It most definitely took place after 1 (note Wild Dog's mechanical arm); how long is uncertain. It doesn't affect anything that happens afterward, so it's no surprise you don't hear about it. Crisis Zone and Razing Storm are unrelated games which use the TC 2-and-later engine.
 * Darius has its own gaiden game in the form of Syvalion. You even have a Silver Hawk fly along side you in one stage and the metal dragon cameos in Darius Burst.
 * Fun Orb's "Armies of Gielinor" is a Turn-Based Strategy based on the history of the world of Runescape.
 * Every Grand Theft Auto game from GTA III to San Andreas was a Gaiden Game; they all took place in the same universe and had some recurring characters, but took place in three different decades (the Stories games took place a few years before Vice City and III) and locations. Other than a few characters who appear in multiple games, the storylines are completely unrelated and don't affect one another. Grand Theft Auto IV totally remakes the universe with a brand new Liberty City, though Vice City and San Andreas are confirmed to exist. How they've been redone is unknown, but will probably be seen in future sequels.
 * Call of Duty 4: Modern Warfare for the Nintendo DS, in relation to the versions released on Xbox 360, PC, and PlayStation 3. The game features similar missions, but featuring characters from other teams operating either in parallel or in support of the teams from the main release.
 * Nie R is one of Drakengard, with the former taking place after the most bizarre ending of the latter (Caim and Angelus chases an Eldritch Abomination into modern day Tokyo and and after defeating it are blown to hell by fighter jets).
 * Not only that, but Drakengard's joke ending becomes Serious Business for Nier.
 * Valkyria Chronicles III is a Gaiden Game to the original Valkyria Chronicles. It takes place during the same time frame from the perspective of a different unit in the same army as the original game's protagonists.
 * Epic Battle Fantasy does this in a similar way to the Touhou examples above- and inverts it. How does it invert it? 1,2, and 3 are basically RPG games. EBF 3.3 is a bullet hell game.
 * Fallout: New Vegas is an odd case of a Gaiden Game that feels more like a sequel to its predecessor (Fallout 2) than the actual sequel does, due in part to the fact that 3 moved the setting to the opposite end of the country while New Vegas takes place closer to familiar ground.
 * Back in the 1990's many PC and Amiga titles were made into a Christmas Special Gaiden Game, usually released in some gaming magazine's cover disk as a present for fans. The games that got this treatment include at least Lemmings and Jazz Jackrabbit (the latter of which actually got two separate Christmas editions, the 1994 "Xmas Edition" and the 1995 "Holiday Hare").
 * Gargoyle's Quest has the Japanese title Red Arremer: Makaimura Gaiden; it is a spinoff of Ghosts N Goblins, or Makaimura in Japan.
 * Aleste Gaiden, in contrast to other Aleste games, has the hero running and jumping in Powered Armor and a relatively limited weapon selection. The ending reveals that it takes place in Another Dimension from the original Aleste, with the same protagonist and villain.
 * The Gundam anime franchise has quite a few Gaiden Games, most of which are spinoffs of the original series and depict events that take place at the same time as White Base's adventures but in different parts of the world. The best-known of these include Rise from the Ashes (set in Australia), Blue Destiny (set in North America), and more recently Gundam 0081 (which takes place between the original series and Mobile Suit Gundam 0083: Stardust Memory). Some other games shift between this and a full-on Licensed Game - Zeonic Front and Federation vs. Zeon on PlayStation 2 alternate between missions totally separated from the events of the anime and missions that put you right in the middle of major battles from the anime.
 * Shift Freedom, which has the same mechanic as the main Shift series, but does not appear to be part of that story.
 * The Ogre Battle series has two. Ogre Battle: Legend Of The Zenobia Prince, a Japanese-only game for the Neo Geo Pocket and Tactics Ogre the Knight of Lodis, both of which tell the backstories of characters from Ogre Battle and Tactics Ogre, respectively.
 * The second Expansion Pack for First Encounter Assault Recon is this for both the original game and its first expansion pack, starting within the last hour or so of the original game and ending before the first expansion does.
 * Colony Wars: Red Sun feels like this in comparison to its two predescesors. The main character is a neutral bounty hunter with no ties to either of the main factions, it takes place concurently with Vengeance rather than after it and the League/Navy conflict is mostly in the background, with most of the missions instead involving feuds between newly-introduced factions and the protagonist investigating an outside threat.