Okay, I've heard a lot of things attributed to Gantz that aren't adequately explained, and then I started thinking back to the very beginning of the manga.
We know that there's a bomb/microchip in each hunters brain, that can somehow up-link with Gantz and detect if they leave the mission area. However, given what we know about the rest of Gantz's technology, I think its safe to assume he can fit a lot more functionality into such an implant.
For starters I wondered why it was attached directly to the brain. I know the Mangaka probably doesn't write every page with Anatomical correctness in mind, however most contemporary ideas for "brain implants" have them mounted on the inside of the skull, not the brain itself. Then I started thinking maybe the chip handles Input and Output signals with the brain. I believe this could explain several previously unexplained phenomena.
If the chip can up-link a hunters position with Gantz, as well as a hunters physical state (IE alive or dead), Than wouldn't it also be possible, given Gantz's technology, to upload a current copy of the hunter to gantz, rather than reviving them from whatever was recorded during teleportation? I know this is phrased awkwardly, however its been argued by a few members here that Gantz records/updates each hunters body and soul every time they are teleported, and that when they die he reloads from the "last save point", however this doesn't account for a hunter's memories up until his time of death, my theory does. If Gantz does treat the hunters like players in a video game, then it would make sense to "save" right before going off to fight a boss or mini-boss, right?
Several questions were raised as to why the door to the backroom had been locked up until Izumi went in to get his sword, however if one looks back towards the first volume of the manga, one would realise that the doors and windows were never locked, but rather they couldn't be touched (Pg 53, Vol1, Ch2).
When one considers that the chips are capable of producing a ringing sensation in one's brain, perhaps it is also possible to directly interface with the brain's motor-skills faculties. All the chip would have to do is send out a few signals to a few nerves/muscles and it would basically be able to nullify a persons attempt to preform a certain action. Perhaps this also explain why they can't physically harm Gantz, or at least haven't tried. This sort of chip-output relates more so to my third and final point.
So lets say you get 100 points and choose option number one, you go free and have your memory erased. While it doesn't go into much detail as to how this takes place, I doubt it completely wipes ones memory, but rather just during the period of time for which one knew Gantz. otherwise Izumi, and possibly Sakata would be left with the mental capacities of infants. However to accomplish this Gantz needs too things: A recording of ones memory prior to knowing Gantz, and a means of erasing and re-implanting that memory, should a hunter choose option number one. I believe the brain bomb chip is capable of both of these, as hinted at in my previous two points.