ok here's another theory
all those missions before were actually Gantz training soldier to prepare mankind for this Katastrophe alien 👽 invasion. And this is what the counter was for
And to be absolutely fair, what Gantz doing is perfectly justified, as those soldier in training were on the verge of dying any way
and this is their 2nd (3rd or 4th) chance
and yeah we definitely need 2 Kuronos now, Reika's decision might not be that bad afterall