You can be rest assured that Gwar will never play the Superbowl. It has nothing to do with their popularity. If the NFL could make money from having Gwar play, they will put them out there and milk it for all it is worth.
No, Gwar won't play the Superbowl Halftime Show because that would scare the ever-loving crap out of millions of old people who are there to see the booby commercials and eat themselves into a coma. Bruno Mars is pleasant and has no edge to him. That's why he is their perfect musical act.