Here is my theories.

The beginning of gantz, some highly advanced ancient civilization first encountered the alien that the gantzer are fighting now, and they fought to the bitter end to drive the alien out.
The remaining warrior than setup a technology and classify them together as gants. They than went inside the black orb which is also part of gantz and started controlling it.

This orb is used to go through the area that they are assigned to find suitable and fitter warrior profile and use them to fight the alien invasion.
As the time pass, and city start to develope the warrior resource become thin, thus they created more orb and used ex-gantz champion to operate them. They continue this cycle of training, recruiting and expanding.

Hoping that at one day they would find the one man that they think is able to lead all the gantzer to invade the alien planet and end this endless cycle of wars, and bring peace to the world ?

Any take on this, just letting my imagination goes wild. =]