I wonder if the entire Gantz Game only operates in the major cities of developed countries. In America's case it would be in New York, Los Angeles, Washington D.C.
Nishi also implied that Gantz operates in Israel as well. Somehow I think the Israelis might have an easier time with Gantz than the Japanese. Israel is a more militarily active nation than Japan (just about all their neighbors don't like them). I'd imagine that an Israeli commando with a Gantz Suit and weaponry would be a force to reckon with.
I just can't see the Black Ball easily hiding in a straw hut.