Gonna go ahead and try to find the source.
Edit:
One of our philosophies is to try to not dictate what's OK or not OK in League of Legends unless we absolutely have to. We rely on data like Reports, Honors, social media, the Instant Feedback System and Tribunal to help determine the standards of what the community believes is OK or not OK. This is why all of our systems are intentionally designed to be community-driven, because we want players to have the ability to invest in and shape their own community. In saying this, looking at the data, "GGEZ" is considered a negative term, and is reportable. Players generally agree that the phrase is rarely used in a positive context, and is a pretty sour way to end a match.
Sadly, the source for this is no longer valid, as Lyte apparently closed his Ask.fm account: http://ask.fm/RiotLyte/answers/132069455298
But this wasn't what I had in mind, so I'll continue searching.
Edit2:
http://boards.na.leagueoflegends.com/c/memes/LkEHlvpI-when-you-have-a-rough-game-and-the-enemy-says-gg-ez?comment=0024
Here's a red confirming the above as a quote by Lyte.
Edit3:
Can't find the quote I was looking for.
Either way, the lack of cases of punishment occuring with only gg ez being used as a negative term, as well as the quote by Lyte support my point well enough, in my opinion, to not be considered "bullshit".