The Day the Earth Stood Still (1951 film)/WMG

Klaatu is also a robot.
What seems more likely: that a race from another planet evolved to be virtually identical to the humans of Earth; or that they sent to Earth two robots, one only vaguely humanoid but indestructible and with a great deal of firepower, the other vulnerable but indistinguishable from an intelligent, clean-cut, good-natured yet serious human -- in other words, an ideal ambassador, and one much more likely to be listened to by humans than a sac of amorphous protoplasm, or a five-foot armour-plated leech, or whatever the aliens really look like?
 * In a loose sense, the short story "Farewell to the Master" that inspired the movie's plot involves Gnut being Klaatu's creator and not the other way round. It could be that, in that other planet, robots are "living beings" and living beings are "robots", conceptually speaking.
 * The new movie seems to support this; in it, Klaatu is played by an actual robot, Keanu Reeves.
 * He could, in fact, be a bioloid representative of the Scarran Imperium.

Or, he's a clone.
It seems sensible that the aliens might not be able to create a robot that looks convincingly human, but growing their own altered human from scratch would explain Klaatu's vulnerability. They also x-ray him early on in the film and find out his anatomy is EXACTLY the same as a human's, so a robot seems unlikely but a clone very likely.
 * Again, refer to the short story's twist.

Gort is the real boss.
This is the final reveal of the original short story, "Farewell to the Master".

Klaatu is a future human.
Klaatu doesn't just look human, he is human. Humanity survives the cold war because of Klaatu's interaction, and eventually builds those neat saucerships, robocops, and temporary revival machines. They go to space, but never find the aliens that rescued them from the brink of nuclear war all those years ago. They develop time travel, to try and observe the events of the movie in more detail, but discover that Klaatu's ship came out of nowhere, and disappeared back into nowhere at the end of the movie. They send back a saucership with a robocop and cause the events of the movie to happen, so that they'll be able to make them happen. It's all a big Temporal Paradox!

The message passed on to Gort is the magic words from Army of Darkness and the whole movie was a Xanatos Gambit to summon the deadites to kill all the humans
Gort and Klaatu were sent to destroy the humans before they made interstellar contact on their own.
 * Ah, but those words would have STOPPED the Deadites. It was Ash saying them wrong that led to the plague getting worse. Obviously, aliens were trying to save us from the deadite hordes until a certain S-Mart employee shows up.

Gort is a God Warrior
From the manga of Nausicaa of the Valley of the Wind, more of his kind appear; and they nuke humanity once they show they can't stop warring with each other.

Klaatu Barada Nikto...
...means "Klaatu lives not." There was a deadline by which Klaatu was supposed to tell Gort not to destroy Earth, and the only way to stop him after he gets going is with a distress call from Klaatu.
 * Not, a pre-programmed codeword for Klaatu forgives? So that when Klaatu dies, someone who was told the codeword could tell a robot to prevent vengeance with the destruction of Earth?

Gort is a trans-human.
Instead of an actual robot, Gort comes from a civilization who upgraded themselves through cybernetics. Klaatu is a cloned meat-puppet he created to interact with humans because he didn't think they were advanced enough to understand the concept.
 * So you're saying Gort is a Cyberman?! Cybus or Mondas?
 * Only less evil.
 * You have to ask? Mondas. Look at his outfit.

The people "back home" don't know about this whole "go around and put races on trial" thing; if they do, they just try to pretend they don't.
Okay, so let's get this straight. You send an Emissary to A-bomb possessing planets and tell them to either stay out of space, get rid of their nukes and join this galactic super-UN, or get blasted to a cinder. Sounds strangely like CIA covert operations, in a way. Who cares if we've overthrown legitimately elected governments, just so long as they don't help our enemies or threaten our superiority!
 * So, kind of like Special Circumstances in The Culture?

Klaatu's people are enslaved to robots like Gort.
His people regret the decision to leave the policing to the robots, but the robots by nature prevent any going back on that decision. While the robots aren't opressing them, their presence is stifling. If the humans would have had enough firepower to actually defeat Gort upon arrival on earth, Klaatu would have been asking for Earth's help to free his civilization from the robots. This is why Klaatu advanced towards the military with a weapon-like device - he was specifically trying to get exactly the reaction he got. The device may have even been a weapon humans could have used to defeat Gort, and Klaatu was lying about its true nature after it was broken. Because the humans didn't/couldn't defeat Gort Klaatu had to just go on about his job like normal, and hope the next planet he is sent to can save them.