To answer the actual question of the post, no saying gg is not considered bad.
Some players react negatively to you typing GG and it is usually when they have lost and are angry, and seeing a GG pisses them off.
that doesn't mean saying gg when winning is considered bad. The term good game is a sign of good sportsmanship, and this is recognised in both sports, and video games, and it is the same for chess.
That's all. nothing more, nothing less. a sign of respect.
personally gg just seems like good sportmanship to be
what do you want them to suck, 'haha you suck?' i think gg is just better