G fares are used for Premium Economy seats, not Upper Class. An Upper Class seat would have a J/C/D/I/Z fare class.
No, G fare is Upper Class for VS fare codes . G is used for award & upgrade inventory in Upper Class and it is what needs to be available to use Sky Miles to pay for UC. Unclear if this is what OP meant. However, G
is the
Delta code for Premium Economy booked on VS or AF on
DL ticket stock.
OP: I would call Delta ASAP to maximize the amount of options you have. They should be willing to put you in either Upper Class or Delta One on any reasonable routing for no charge.