Well, here what we know:
There is more than one Gantz team - others exist in other parts of the world.
Gantz only revives people and makes them join only after they are "killed." Yet, civilian casualties are in the hundreds in the last two missions.
The enemies they fight are "aliens," that live on earth.
Nurai says that everything is an act of God and their world was made by God.
All the rules of the "game," basically state that no one must know about it - yet Gantz reveals it in the last two missions.
So, if killing these aliens is somehow important to Gantz, using normal people in super suits would not be the most effective way of going about it.
Thus, it leads me to believe that the Gantz world is somehow created for the sole purpose of entertainment. The aliens are probably there for the same reason normal people participate - to get slaughtered. It's like an intergalactic gladiatorial ring, where living beings duke it out.