After some thought, I believe that GANTZ, was sent as from the future as a way of preventing the cataclysm that would happen in the future. This "end of the world" was mentioned via Nishi's website on GANTZ. It is quite apparent that the destruction of the World is due to the aliens.
This would explain:
1. The Advanced Devices in GANTZ's room, such as the ability to clone, have guns, supersuits, etc.
2. The purpose of hunting aliens
Why would GANTZ want to eliminate anyone from outside his game who watched it?
1. Because said person might leak information about the gantzers, thus making GANTZ's struggle, futile.
What would explain the vampires, though?
A. The nanomachines that were made to make people into vampires was sent back from the future, as a means of destroying the GANTZers because of a conflict of ideologies. An example being "Human Scientist A wants to save the human race, but Human Scientist B profited from the destruction of the human race. Therefore, B tries to stop A's Technology(GANTZ), by creating Countertechnology(nanos)"
B. The nano machines were created by Aliens as a way of keeping the future as it will be.
Why the randomness in GANTZ's new recruits?
A. GANTZ may have been damaged on the way back, thus getting useless people as well as strong people.
B. The person who created GANTZ didn't care who died as long as they killed the aliens.
I'll add more to this as I think it up... but yeah. Those are my theories.