Since Fallout 2 isn't available on GOG anymore, what are you wanting?
I thought you can gift a game if you hasn't downloaded it.
You have to mark a purchase as a gift when buying on GOG.
If the game is on your shelf, there's a chance that GOG staff can convert it to a gift code as long as you've never downloaded anything from it. You'd have to contact them for that, though.
EDIT: I doubt they would convert Fallout 2 to a gift code in this situation.