I think gg is indeed a sign of sportmanship. Because you also have 'rage quitting'. That is quitting/resigning the game very suddenly without saying gg or giving some other kind of sign. I used to play Starcraft (2). It's a VERY big esport 1 vs 1 strategy game. (In Korea it's even bigger than soccer in Europe, with tv channels and pro-gamers that are worshipped like famous rockstars). In Starcraft it's normal that the losing player says gg before he surrenders. In that scene there is a pro-gamer called Idra who gets bashed on the internet alot because he never says gg and ragequits all the time.
So in my experience is taking GG as an insult ridiculous.
This blog is the best argument I've seen yet for turning off chat. gg (Good Grief!)