Just an idea, but could the bets have affected gantz behaviour?
For example gantz seemed to hate katou a little with always teleporting him very late, and gantz seemed to have a thing for kurono action (chibi alien+roman arts hunt, in which kurono was left alone by gantz vs the david statue), so mb the betters hated mr i save the weak because they wanted to see more ppl dead, and they liked kurono for obvious reasons.
But how does this...yea, this destruction of the sense of gantz (i think most of us thought gantz was at least useful regarding the defense of the earth) fit with nuri's god thing and the vampires?
And if the aliens are man made, why are they so ridiculously strong?
I could understand it if the aliens would be killed off even after failed missions, but the chibi hunted kurono down in his school, and i doubt the betting organisation would have such a risk occur with for example the oni aliens.