I too haven't seen any game rules that went by GG. Would be too confusing with Good Game, as mentioned about. I've played here and there, so I may be wrong, but assume that GG = Good Game.
I am the Carthaginian who became an angel, and surrendered his wings for a life on the sea of battle.
My magic screen is constantly bombarded with nubile young things eager to please these old eyes. This truly is a wonderful period in which to exist! - Terikel the Deflowerer