If they won: gg. Translation: good game, well fought, deserve it, etc.

If I win: gg. Well fought, I'm humbled to play against you, better luck next time etc.

If a glitch occurs or something weird happens: gg. Translation: Wow that was pretty lolsy.

If they 'cheat' or 'play cheap': gg. Translation (situational): I kinda pity that you had to do that and am disappointed that you did but I also acknowledge that I could have done the same and was also unable to stop/prevent/counter it. And if it starts to annoy me for whatever reason then just leave and rest assured that your shopping list will be that bit cheaper since you don't have to add extra salt to your shopping list.