So, I've been playing games online for at least 15 years now. When I was younger, I played soccer, football, etc. At the end of every game, online or not, I've pretty much always said 'gg' or its equivalent. I was taught to when I played my first real sport. We lined up and shook hands. It sucked if we lost. Especially if we lost hard. But you know what? The other team played well. So, we showed our respect by acknowledging that. And if we won? No reason to stomp them while they're down. So we told them 'good game', and maybe next time they'd beat us.
I'm always surprised to see people so upset over people saying 'gg' at the end of a game. If it was a really good game, I'll usually say so. 'gg, wp, that was a lot of fun' or something. If someone disconnects on their team? 'gg, sorry for your disconnect'. Just because they had a disconnect doesn't mean that I need to say they played badly.
Everyone is welcome to their opinion here. Personally, in all my time playing real games or online games, I've always said gg. I feel it's better than saying nothing at all, and considerably better than verbally abusing whoever I just played against. Your mileage may vary.