If you went and lost a game of soccer, would you tell their players it was a bad game? Sportsmanship, congratulating the other players, that's why you say GG.
the way i see it, gg is just good sportsmanship, like shaking hands after a sporting match. no one is forcing you to do it, it's just good manners. if i actually did think a game was really good, i'll actually chat to my opponent with real words that form sentences :)
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