I'm studying Andate no. 6: in 'Studi per Chitarra' by Ferdinando Carulli
but I can't figure out what the '?' chords are. He could be syncopating but I feel that is unlikely because this was written sometime in the late classical era.
It's basically a V or V7 chord, but using F# as an auxiliary tone to intensify the G.
It doesn't really represent a chord at all. Not every note in a piece needs to have a chord assigned to it. Where did that notion come from?
Although the F♯ doesn't in fact belong to the prevalent G7 chord, those F♯s are there as an embellishment of the G notes. In much the same way as the motifs in Beethoven's Fur Elise.