What else but Gabriel Knight 3?
Of course aside from its main theme it also has some exquisite piano tracks, especially the two Montreaux's themes (and especially #2 (in the sound files named Nocturne Fast), but (I'm guessing) good luck learning that without notes). And, well, any song from GKIII really...
(I've personally always wanted to play some of those GK things on the guitar, but it's just too bad I'm not too good at copying melodies without any notes)
Or, if you want older, any other Gabriel Knight. Sins of the Fathers should at least have some very notable tunes.
Not sure how well they translate to the guitar (I think the GK3 main theme should at least work quite well as more of a ballad-type thing (or if that sounds silly, I mean slower))
Aside from those... Didn't Beneath A Steel Sky have quite a good theme? Morrowind did. I'm not sure if that would translate to the guitar or not. (Edit: although I'm sure they've been done a once or twice at least...)
(I'm just throwing ideas here, it's not like I'm requesting that you play all of these. I'm sure you have something else to do too
)
