It sounds like Spamalot is a better choice than I thought, then. You need to add those challenges to the example, and perhaps note just how serious or silly they are (and if they count as straight implementation of the trope despite the comedic context, or as a deconstructive parody).
As for the Gaiman story, without actually reading it myself, I couldn't say exactly. From the summary you wrote, it seems to me that Mrs. Whitaker is, in and of herself, the challenge to overcome -- the various things Galahad tries are not precisely challenges in themselves but rather part of the greater challenge. As for a "Grail Guardian", well, traditionally the Grail is its own guardian -- it goes where it wills and does what it wants, regardless of the mortals around it. The closest thing it has to a "guardian" in the original stories is the Fisher King, and he basically just offers another challenge in the Quest (a challenge of wisdom, prompting the knight to ask the question that will ultimately lead to enlightenment and the Grail).
But don't get me started. My wife and I wrote a book on the Arthurian mythos for Steve Jackson Games coming on thirty years ago and we're still both very well versed in it. I could go on for hours...