GG stands for Guaranteed Gains. A creator who is already in the business for some time, or a professional agency, use a trial link and promote you and your account to gain a specified number of followers. This typically applies to the OnlyFans website and it's creators.
Ken: Hey, can you help me promote my OnlyFans account?
Barbie: Sure, pay me $300 and I will help you get a thousand GG's to follow your page?
Ken: Really? That's cool!
Meaning "good game" this can be used after you've played a game. It could be after a Bored-game or a sports game. You use this in a positive way.
Gg that game was good, nice goal last minute
Either said at the end of a match. In any game.
Wins 1v1 10-9.
Me: GG.
Him:Stfu you're bad.
Someone at school shits his pants:
Us: GG, G-FUCKING-G
Him:Please, stfu.