The problem here lies in the fact that there really is no way to prove if someones "GG" was meant in a respectful way or not.
In the heat of the moment, when you die, you may feel frustrated. It's only natural, especially when you consider yourself good at the game like I did for a while. In this moment in time, to some people, it doesn't matter what the person who kills you said. They could say, "That was the most fun and intense fight I've ever had, you were a very skilled opponent." but you'll still be frustrated at them, so they'll interpret that as they want. (I.E, "oh, look at him gloat over his skill, he thinks he's better than everyone." This isn't hard to interpret from that phrase.) But can staff ban for that? Heck no, it wouldn't make sense to, the player likely didn't mean any harm in saying it.
The same applies to GG in a way: If someone kills me and says GG, of course I'm going to be frustrated...maybe not even just because they said GG either, but it certainly didn't help. Sadly, there is absolutely nothing the staff can do about it, because there is no way to prove any harm was meant. When I discussed this briefly with my brother, an avid LoL player in the highish tiers, he honestly didn't get how GG was seen to be as disrespectful as it is seen by people like myself, thus if he played here frequently, he might be one of those people who would say it with no ill meaning...He does not deserve to get in any trouble for his ignorance, unless of course it's clearly stated in the rules not to, which at this point it's not and honestly it shouldn't be.
As of recently, I've heard of numerous occasions where people interpreted GG to be "Get Good" instead of "Good Game". Likewise, I've heard of people who use GG to mean Get Good instead of Good Game, thus causing controversy even when used by the losing player. GG is just a stupid thing to say at this point in my honest opinion, there is way too much controversy and misconceptions about it that it ends up causing more trouble than it causes good...but this is the same reason why it can't be a banned phrase, because some people truly do have good meaning while saying it, and they don't deserve punishment for it. I just really wish people had a better understanding of the harm it causes.
It's people who say GG after they win fights that have caused people like myself and Gravey4rd (especially Gravey4rd) to become stressed over the game and, at some point in time, leave. People really underestimate the power and meaning a simple two letter phrase over the internet has, it's not something you just "get over." Honestly, the way many (not all) of the better players and players in clans act after killing someone in a notably luck based game disgusts me, and people saying GG is at the bottom of my concerns with that...the worst part is that most of these people are just kidding around when they say it and don't realize what they actually said. "get rekt" is a terrible thing to say. I don't care how serious you are or not, it will
not convey through text when the person you are talking to is likely not happy with you already.
Sadly, my constant worrying about these things myself has ruined my own experience in the game. In my case, I hate to make people become frustrated at me over the game, but the reality is that it doesn't matter what I say to them, they'll still be frustrated at me if I kill them. If I say anything that isn't very, very carefully worded, they'll see the worst in it, and if I say nothing, they'll assume even worse than that. Such is the flaw in long games that take many minutes to complete, which sadly cannot be fixed. It's part of the reason people like smaller maps.
If there is any one thing I've learned in my time in online gaming, it's that fame does weird things to you, and it's not worth your time unless you're willing to put up with some serious amounts of...well, not fun stuff, such as the GG's discussed in this thread, and I know this having never been truly popular. Sadly, human instinct keeps telling me to strive for some form of fame, to be the best at something. It's things like this that make me respect people like Youtubers -- especially the big ones -- that much more: I can only imagine the stuff they go through, considering it can practically be their job...
Between the rude things people say and my own ego, I've seen the worst of myself when I was at the top of MCSG's very own leaderboards and on it's moderator list, and I have no intentions of ever going back to either of them. (Sorry staff D

There are some awesome people up on MCSG's leaderboards, but I'm not one of them. I'm sure there are plenty of old top players who know exactly what I'm talking about, though there may be equally many who never saw this in themselves, or never had it as a problem. (keyword: old. Like, long time MCSG player old, not age old.)
I could go on for ages about this topic, but I feel I should stop here. I just wish people understood the true volume of the problems caused by simple bits of disrespect, even over the internet, and yes, even in two letter phrases. It's more of a problem in today's online gaming industry than you guys might think it is.