I was thinking what if Gantz is some kind of weapon or automatic defensive system or research probe from future or deep space, but what if it was forgotten by itís masters or constructors and it goes far from itís original program. So Gantz is half-dead system whit only partial functionality of some modules. Thatís makes all sense: why Gantz have pretty sic but humanlike sense of humor but it canít see the difference between human and animal? Also that explains why Gantz does not tell anything about weapons or suits and other unlogical actions if tge Gantz. Maybe itís just half-mad hi-tech computer system?
Just like, you know, some kind automatic factory even if there will be no parts to construcs something automatic manipulators will still catch thin air until all power is gone so maybe Gantz in some way the same?