"And I don't have a score to hand to see what the composer notated. It might well work to follow whatever happens there on guitar - as per usual, if it sound right it is right!"
In the first measure of Gnossienne I, for instance, the grace note is written with a slash through it, making it an acciaccatura--played before the beat. In this first measure, the A in the chord with the grace note (which is from E to D sharp) would be played on the beat, along with the D sharp. I suppose you might still say that the grace note could be played so quickly with the D sharp (as a sort of "crushed note") that you could still play the grace note and chord together and the A would sound like it was on the beat. Still, I know how I'd play it on the piano, and I don't see why it would be played any differently on the guitar (meaning I think the grace note would be played before the beat and the bass note on the beat). A very interesting discussion indeed.