Is Gantz answerable to someone or something else?
I have been thinking about it recently and i believe the answer is yes. I don't think Gantz sets the rules or at least not all the rules. So i believe that someone else sets the rules for gantz to follow and the only time that he avoids the rules is when he finds a loop hole.
My point is that Gantz only takes people who have died recently as new recruits. His technology is obviously of the level that he could take anyone and anything he likes into the gantz room but only has taken dead people. He even goes as far as i encouraging Izumi to kill people specifically for gantz recruitment. He even requires that Izumi, who would willingly participate, must die in order to re-enter gantz. So obviously Gantz is not calling all the shots.
The only people who entered gantz without dying are the vampires and they did so by using a loophole.