Gantz is just a game so, we have an alien (gantz) bringing to earth some more aliens, varied ones, so I think from differents planets, that’s mean less powerful than gantz itself (that way he can manipulate them esaly) and then he create a “Gameboard area” introducing this aliens, exception is the vampires but they know about this game and they want to stop it. And then he put a group of persons on these missions, of course varied one for really fun (what fun if all were killed inmediatly) so a supose always there is a winner or will happen when Kurono lost on game and the alien goes to hunt him. But, IF REALLY this is a game like aliens vs predator, so, should be more Gantz out there on earth, and also in other planets, what they win with this game? Is a competence? Who kill more aliens? Thinking the humans ones are not importants, just mere pieces of this chess just like the aliens.
But the way, sorry my english