Looking over the past missions in Gantz, I have a theory. Firstly let me summarize the past parts of the mission that will help my argument.
1st mission, they hunt a father and a son in their own turf. (I assume the house the Onion alien was in was his.) Also notice you cannot see them.
2nd mission, the birds have their own base, and actually seem to react with the humans peacefully (when the Tanaka stole the eggs from the store, and the man commented on how funny he looked.)
3rd mission, Buddha's also seem to be in their natural habitat. Same with Shorty mission and the dinosaur mission.
But the Oni mission, things got different. The aliens came to the Gantzers in massive numbers, all seeming to be led tactfually by a small elite. The new mission seems the same way.
So my theory is that the aliens were normal citizens, maybe immigrants to earth, and hid themselves to live at peace with humans. Gantz, maybe from another organization or human technology (Don't have an idea for it yet) finds out about the aliens hiding away, and sends 'men' to exterminate them, with threats of death if they don't fulfill their missions correctly (Kinda like the gestapo or something). Now the Alien motherworld or planet is pissed at their citizens being killed; so obviously they send armies to take over earth, and no longer need their aliens to be invisible, because they don't need to make peace with the humans. The vampires, i believe, have some sort of connection with the aliens (the oni mission when they brief the boss about the gantzers). So now the aliens know who the soldiers are and where to attack.
This is just a theory, but i believe i have some reason to think this true.