Well i say GG ,after the game.
I find it unfitting if the game is still in progress and only one player fell.
GG means the whole game.
If the game was a joke i tend to say nothing especially if i feel there was no real competiton within the game.Usually there is atleast a minimum feedback i give.
I for example ,would have real trouble saying GG after getting unfairly tractored, plus to peek it, get moved in a rock.,not that that ever happend to me..,but nonetheless.
The whole thing is depending a bit on what you consider a GG.
If not that ,i would say Thanks for the game,without label..It depends a bit on your opponent too ,of course.