If people blame GTA for misbehaviour, that's bull, and I'll tell you why. If GTA was harmful, it would make you do things without you knowing why. It would be unconciously, so you wouldn't have a clue where you picked it up. If you say GTA made you do it, you clearly had the intention to do it, and you were only inspired by GTA. There is no difference between GTA or movies or even books then, because the choice is ultimately yours.
It is however possible that GTA screws up certain peoples' unconcious behaviour, but I can't say anything about it because I am not a psychiatrist and there hasn't been any fundamental research about this that I heard of.