Short for `good going` or `good game`. Used sincerely in online games such as trivia, but more often used sarcastically when someone has done something foolish. Also, it is now becoming customary for all players to say `gg` when the game is over in such games as Counter-Strike , unless a player is profoundly displeased with the outcome.
Found on http://www.ruf.rice.edu/~kemmer/Words04/usage/slang_internet.html