Yeah, gg is used at the end of a match but its also used very sarcastically.
Ex: someone goes afk for 1-2 mins in a dota match and people will cry GG. Or you make a game breaking mistake, and you can say GG, because the game is pretty much over (so it kind of means epic fail as well as good game). It seems like only starcraft players really use it for its literal meaning, mainly because its a 1v1 game.
But yeah, that's what's so awesome about it and also why this keycap is going to be so great!