Methinks good demeanor is what keeps your gaming experience enjoyable even if you're gonna lose the match. As a multi-player arena game with such gigantic player base, it's unavoidably you'll constantly come across with someone with bad mouth, and these include some very skillful players. What I want to express in this post is that, you may want to act like a nice guy, but sometimes your tongue just slipped that might cause misunderstanding.
GG (good game) may seem to be an appropriate curtain call, but it's only appropriate when both sides have fought an uphill battle against each other. When my opposing team has got an incredible noob/feeder/troll/DC who leads to their demise, I prefer not to /all GG or just leave GG to my team's channel privately. If I really want to praise my opponents I will say "GG both" so it doesn't seem too smug.
I did learn it since I was surfing my first internet forum from its admin: if you can't say something good, don't say it. Literally never say BG (bad game) or alike, it never helps. Blaming your teammate noob just make yourself a noob too because one can't improve themselves instantly just because you label them noob. If you feel terribly apologetic to your teammate for the defeat, depending on their reaction to your mistake during the battle, you can say sorry, or sometimes remaining silent would be the better way to go.