A Few Theories. (Off The top of my head, mind you.)
1. I read on Wikipedia that there is speculation that there may be other Gantz teams out there and one central sphere in Germany. So, maybe every once in a while, they all compete against each other to determin which one has the best soldiers. And the losers die or something like that.
2. If you think about it, Kato would look exactly like Izumi if he didn't slick his hair back. Maybe, since you get your memory erased when you leave Gantz and Izumi's already played before. Maybe When Izumi died the first time, his original body didn't die like with Kishimoto. Remeber, Nishi said something about another guy who that happened to. Maybe it was Izumi, or Kato, whoever he was originally. So, if it was Kato originally, and that would make sense because he has a family that you see and Izumi is living by himself, maybe when the copy, which would be Izumi, woke up from having almost died, maybe he had some sort of amnesia and made up a new identity for himself. But then there's that fact that Kato hates killing, so maybe he's the copy and Izumi still had amnesia when he appeared in the room. Anyway, that's just a theory.
3. (Spoiler alert.) Since Gantz dissapeared at the end of the Anime, that may help my first theory. All of his soldiers were dead (We Assume.) So he was instantly out of the running so he just dissapeared or was sent back to the central sphere.
4. Maybe Gantz is an evil alien. And the other aliens that the hunter's kill have been sent to earth to Destroy it. Gantz is trying to take over the world, so it has to eliminate all threats to it. And it has to work in secrecy so it made Tae and mission objective so it won't be found out by the normal humans. However, that doesn't quite answer why humans can now see the Gantzers.
Also, Does anyone know why the timer and the cloaking and all that just turned off? And since people can see them now, are the Hunters aloud to talk about Gantz to other people without their heads blowing up?
How can Gantz Fit inside the sphere. It's okay when the guns and suits come out, but when all that stuff is back in there, how is he not crushed to death?