I think it's more the whole retarded-@ss mentality that video games are just for kids, and the fact GTA has been around longer. Since GTA has been around longer, more people are just familiar with it than God of War. If God of War came out back in 01 or 02 and GTA came out recently, I'm pretty sure the stupid people would be b!tching up their little temper tantrums over that then GTA.
Look at Columbine, the morons were saying Doom teaches kids how to fire a gun. Yes it was the game those two liked to play, but there are far more realistic FPS out there. I don't recall anyone b!tching up a storm over the then new GoldenEye or MoH.