So true. Unless you say it after a loss, or a 60-minute back and forth game. Then its just funny.
what is wrong with saying Good Game Ezreal? 
I couldn't agree more. It's super disrespectful and sadly people intend it to be that way.
Another thing that drives me bonkers is people on your own team who say "gg" well before the 20 minute mark. It honestly tilts me hard when people just spam gg, gg, gg, gg its over, gg ff @20, gg you guys suck, gg this team...
like... https://media.giphy.com/media/3rgXBF2ReXLuUpqPeg/giphy.gif
I personally despise the enemy saying "ggez". It is really quite disrespectful. I don't mind most forms of bm, but that one gets me. It bothers me a lot more when it comes from a player who contributed almost nothing more than an extra body on the rift to his team. It's pretty easy to tell if it's meant as a joke or as an actual form of bm. Being told "ggez" after making a single mistake to cost you a game is quite possibly the single most frustrating and salt-inducing thing I've ever personally experienced in LoL.
saying GG after a one sided game is still stupid as hell
I say it irregardless of how was the outcome of the game, as long as i see that victory screen I'm fine....