When you realize that Hoi Hoi is more than just the Team Pet: It's a freaking bear in a strength-enhancing suit. This is confirmed when it manages to rack up 40 points behind everyone's back in the Osaka arc.
Everyone jokes about Tae being plain and homely, but after Katastrophe it becomes extremely apparent why she's a perfect match for Kei: she survives.
The Osaka Arc's storyline was to demonstrate how the Osaka Gantz team, all acting for themselves, ended up dead (see Team Spirit below), while the Tokyo Gantz team mostly survived. Until you recall that earlier on it was established that multiple members of the Osaka team cleared (in essence, "won") the Gantz "game" multiple times, choosing to stay and keep killing aliens because they were bad people for the most part. And one assumes they didn't just adapt this "every person for themselves" attitude recently. So either the Osaka Gantz team went on a lot of nerfed missions, or their luck ran out big time when we finally meet them. Or they died of Plotus Neccessitus. Probably the last one.
One possibility is the Nura alien couldn't be killed via direct attacks (presumably how the Osaka group had done Gantz hunts until then), it would just adapt and become stronger: you had to shoot it when its back was turned in order to do any real damage.
From how it looks, they've cleared the Gantz game as many times as they had before the shit hit the fan (more and aggressive aliens, they're all visible, and several characters noting that it's never been like that before) and if you let it get to your head enough (like how it has for the Osaka team, Nishi, Izumi), you could get arrogant, careless and stupid.
That's the thing: the Osaka team was essentially composed of a core group of badasses: George, Kuwabara, Knob, Kyou, and Oka. In combat, they're shown to wipe the floor with everything short of the Nurarihyon and the two 68/71 point aliens. Kuwabara was able to not only easily dodge the attacks of an alien that had killed no fewer than five suited hunters with one hit apiece, but he did so A) without a suit B) stark naked and C) while raping another alien, at least initially. The worst bit of fridge logic for this troper was the bonus chapters depicting the Osaka team shortly before and after the missions. The team basically just being normal people outside of the hunts worked well: it had an almost Taxi Driver sort of spin to it. They all function like healthy citizens possibly because they have this outlet for their more psychotic tendencies. But that fell apart in the aftermath, when it was down to four hunters, three of whom had over 100 points...
First, there was Miho, who was Knob's girlfriend. She chooses to free herself with her 100 because it isn't worth sticking around with Knob dead. Fridge Logic: why didn't she just bring him back with her 100?
Next, there was Sumiko, who had a tearful goodbye with Miho before choosing to free herself as well, even saying that she hopes she remembers Miho when she returns to her life. Obviously, since the terms of release include the clause "with your memories erased," that won't happen, so that's another 100 points wasted.
Finally, there's Kuwabara, who leaves simply because everyone except the new guy is gone. He apparently forgets that his apparent best friend was decapitated by an alien and that he has the ability to bring him back to life. The entire sequence was like watching a military unit sustain a handful of easily-treated casualties and then, instead of actually treating them, deciding that there's no way they can go on and using all the morphine to overdose themselves and leave their wounded comrades to certain death.
What was the point in Izumi going on a shooting spree in disguise if he was just going to let himself get killed anyway? I mean, he obviously did it to protect his identity, but what's his identity worth to him if he's going to die anyway?
It's on the off chance that Gantz picks him to bring back. Gantzers are still allowed to live their own lives between missions.
Also, Gantz might (and, as it turns out, does) choose victims from the shooting massacre to revive as hunters, and it would be awkward at best if some of the new members recognized the other new member as the guy who committed the shooting. He was counting on Kurono to keep his mouth shut about it too (not entirely unreasonable, since the reason Kurono knew was that Kurono had the chance to stop him beforehand and chose not to).
When Kishimoto (and, much later, Kurono) are duplicated by Gantz' teleportation beam, their duplicates are treated as full-fledged human beings with as much right to exist as anyone else. The Fridge Horror comes in when you realize that this means that every time Gantz teleports someone, it is not actually transporting the individual through space. It is killing the original, and creating a perfect duplicate at the new location. Not that this is out of character for Gantz or anything, but still.
The God Aliens revealed that the game-like aspects of the alien hunts were completely fabricated by a German businessman in order to exploit Gantz as a reality show for the rich and powerful; the aliens only provided the military technology. This means that all the destruction of the invasion was completely unnecessary and the product of corporate greed - if the German who recovered the technology hadn't made it into a game and instead proliferated Gantz and knowledge of aliens to the world, humanity would've had no issue dealing with the giant invasion. Imagine what might've happened if every man, woman, and child on the planet had access to a suit and a z-gun!
