Gantz is a cruel, cruel 8-ball, he is.
It's pretty obvious that anyone good enough to get to 100 points and get out is never going to be "free" in any way we'd define the term.
Think about it: you wake up with, as it seems from Kuronos reactions in 221, no memory of anything you've done for months, or of people you've met. They still fully remember you, of course, and will probably try and contact you, giving away the situation even more.
Changes in your life that Gantz doesn't even pretend to try and hide (where'd I get all this new muscle mass from? And come to think of it, when did I learn to use a samurai sword? Hmm...) would also start cropping up fast.
There's at least one website out there that has the whole story written down, and you KNOW you'll stumble upon it sooner or later, especially once you start suspecting something's up. And I guess that website'll start updating again, now that Nishi's alive.
At least at this stage in the Gantz game, and for all we know, before now as well, every Vampire in the city has your face on his cellphone. THEY don't know you got 100 points. If you survive the first couple of superpowered attempts on your life, by luck, those fighting skills you probably still have, or by another member of your old Gantz team looking out for you and saving your ass, you are going to want some answers.
And of course, the primary reason: Gantz is an ass. See: Izumi. Who just happened upon a baby Gantz in his underwear drawer one day telling him to kill people. Whoops.
Basically? You're in 'till you're dead.
Or, err, you die first, and THEN you're in 'till you're dead. Again. Unless someone brings you back, in which case... no, forget it. It just gets too confusing after a certain point to be worth the effort.