See the problem with 'GG EZ' is that no one likes it, except the users themselves. In ANY game, saying 'EZ' is just the hallmark of being a douche in-game to those not on your team, regardless of how 'good' you are.
Scenario 1) Game has gone over 60 minutes, teamfights are very tied, objectives being taken on both ends of the game. After one fight that took what it felt forever to finish, the red team messes a key play up and they all die. Blue team rushes in and takes their base, spamming 'EZ', even after getting absolutely beat sometimes, and having such a long game, and having a hard time winning at all. Red team gets upset because everyone on the Blue team said 'EZ AF'. Why? Because they had fought for so long, and so tiredly, they made a mistake (remember, humans are things), and your team being your team decided to rub it in against them, and make them feel bad because you took it like it was 'EZ' (even though you knew it wasn't).
Scenario 2) Blue team has 2 people AFK after going 0/3 in their lanes, Red team pushes in hard because there's little pressure in those lanes, team starts collapsing and since they couldn't catch up to cover for the missing spots, they lose. Cue the 'GG EZ'. Blue team gets really upset because they were at a disadvantage and yet you decided to mock them for it, bragging your cheap Victory in their face.
Scenario 3) Bot lane on both sides are matched up, fighting to the death, heals going around, flashes too, Red team makes a mistake and dies to Blue team's ADC. Blue team's ADC flashes mastery and says 'EZ' even though they almost died themselves, and using both summs to barely survive. Red team's ADC respond in a negative way because they think you're being toxic, and understandably so. They start playing a little worse, focusing more on answering to the childish behavior from the Blue team (which is a mistake to not be made), and die more and more, everytime having the Blue team's ADC spamming mastery, repeatedly saying 'EZ' in chat. Things go badly from there, lanes start messing up more often, and Red team loses. Incoming the 'EZ game'.
See the thing is, if you know the game was tough as hell, or was a cheap win because people refused to play as well as they could or wanted to, or because you decided to screw up someone's gameplay by being unsportsmanlike, then you don't say 'EZ'. Ever. I sometimes see people saying 'EZ' in ARAM matches, where some team goes full assassin/tank and the other ends up with all squishy targets, and they get completely steamrolled not because it was 'EZ', but because it was one-sided. Or in gamemodes that are supposed to be fun, like URF, or Nexus Siege, or One-for-All.
TL;D: Don't be a dick and say 'EZ', because even though you felt it was 'EZ', no one likes people that brag/are unsportsmanlike. I, or anyone else, is not entitled to what people can say, but it'd be really nice to not have someone that went 5/12/4 say 'EZ' at the end after getting roflstomped by that Soraka mid, to then so happen to pick up a lucky Double/Triple kill, and proceed to call everyone trash as they did.
Instead of being a dick, suggest some tips to those that seemed to have a hard time, and tell them about those times they did really well, and to practice more to get better. And when I mean suggest some tips, I don't mean suggest them ways on how to commit suicide, for the love of everything good in this world.