GG
It's usually pronouced "gee gee" for Good Game, signalling the end of the game with a concession from whoever uttered GG. As in...
Good Game GG
Sorcery (Rare)
As an additional cost to play Good Game, remove the top 1000 cards of your library from the game and tear Good Game in half.
You win the game.
Sometimes, especially on Magic Online, rude players will use "gg" as a ruse to connive an opponent into a potentially bad play, such as swinging with everyone into an opponent at 1 just to lose to Captain's Manuever.
It is also bad sportsmanship to declare "gg" to imply "please concede, I am clearly the winner".
In short, you should only declare "gg" if you are indeed conceding the game in a sportsmanlike fashion, or as an after match prompt for a handshake when you are the winner.
"topdeck nothin mise well say gg amiright?"