Insulting internet slang for "Good game. Easy". This is a common form of trolling (usually termed griefing) when you beat someone in an on-line game, indicating that they were not, in fact, a worthy opponent.
This phrase is currently in the news as Blizzard has started replacing it in the multiplayer video game Overwatch with various alternate phrases. Users typing "gg ez" will instead post a random phrase such as "I feel very, very small... please hold me...", "It's past my bedtime. Please don't tell my mommy.", "I'm wrestling with some insecurity issues in my life but thank you for playing with me.", "Ah shucks... you guys are the best!", or "I'm trying to be a nicer person. It's hard, but I'm trying, guys."
Common variants of the phrase include 'gg wp ez', 'gg ez noob', 'gg ez no re', and 'gg ezpz.