I doubt GWAR would be on Mayhem, and if they were, they would be stuck with some lame second stage half hour slot.
They only headlined SOTU once, the other times it was crappy half hour slots, and SOTU was a much smaller festival. I also have a feeling the makers of Mayhemfest would not put GWAR on there were GWAR to do their typically stage show, I realize they put Cannibal Corpse and Marilyn Manson on before, but it seems like a bit much when they are marketing the festival to a younger audience.
But hey, they have done festival tours before (SOTU and there was another back in the 90s, can't remember off hand) and they did Wacken as well as some other huge festivals recently so I suppose it's not out of the question. I can see them fitting more in at Ozzfest, but I don't believe they have the best relationship with Sharon & Ozzy.