1) Phrase said at the end of a match, either online or in person. Means "Good Game", and indicates one of two things: Either it was truly a good, fairly played and close match, or one team got slaughtered.
2) Used as an insult. Either in the aformentioned way, or used to insult someone who has preformed an incredibly stupid act.
1a) *Red team wins, 50 to 47*
Red Team: GG
Blue Team: GG
1b) *Blue team wins, 50 to 7*
Blue Team: GG!
Red Team: Shut it.
2) *Doctor Olayer falls off edge of map.*
*Guy accidentally dumps mustard on the shirt of a cheerleader he likes.*
Friends: GG. G-Fucking-G