Not saying 'Gg' doesn't necessarily make you a bad person. Like almost everyone stated, it is just a form of respect towards your opponents after a round of play. Some people, as a matter of fact, prefer not to say 'Gg' after a match, even if they win.
So, it's actually just up to you whether you want to mention 'Gg' ingame. I don't think people would judge you because of this.