What does gg mean?
The term GG is the acronym for "Good Game" and is used especially after each multiplayer game in video games to congratulate the other team or players.
This term serves to indicate to the rest of teams or players that the game has been good, with no drama and has been fun.
How to use gg?
The best way to use GG is at the end of an online game or match. It is usual to write "GG" in chat when you finish playing to inform the other players that you are quitting and not coming back (for example in Poker) or simply that you have enjoyed yourself. It is a term used as a courtesy to the other players.
Usually people say "GG" when they quit regardless of whether they are winning or losing. The rest of the players usually answer with another "GG".
Conclusion
The term GG is used to congratulate the opponent and the other players after a game. It is the acronym "Good Game". It is a common expression to describe and say goodbye to a game.