There are Gantz spheres located in various places around the world. This is where the recently deceased are brought by the local cyborg inside a Gantz sphere, given weapons and Gantz suits, and then sent out on missions.
The Gantz sphere is in the main room. One side room has two monowheel vehicles and a pile of swords.
The only Gantz rooms shown in the series are the one in Tokyo and one in Kyoto.