Dude, what leakages are you exactly talking about? the cat's been out of the bag for god knows how long now. Gantz might have tried that before by forcing Kurono and the other Gantzers to kill Tae before but that was before the Ikebukuro Arc. Since then, everyone, I mean everyone (the media, 2ch and a bunch of others) have jumped on the new phenomenon known as the guys with the black suits. A large number of people died in the Ikebukuro and Osaka arc so I don't see what's left to hide (except Katastrophe, but hell, not even Kurono and the gang know a lot about it).
and besides, is Nishi really the type of guy who does things on a whim? well, with the exception of committing that massacre at school, he's been the same selfish jackass we loved and hated. He didn't even bother to go in-depth about the Katastrophe to the other current Gantzers, what makes you think he'll spill the beans on a complete stranger?
Good point, your right...the rumors of him killing the cat should of got his head blown up but thats a whole different issue like maybe his bullys seen the cat and him outside so they were adding up conclusions for the whole class (Im guessing you need to be caught red-handed for Gantz to terminate a player) and he wasn't seen by a witness during the kill, only after the cat exploded, he was seen next to the dead cat, well thats not the point anymore.
The Government would have probably censored any information, media, news on it after the 'Live Incident' so the public are still unaware of Gantz. I'm guessing theirs a Secret Government Body hidden which starts the Katastrophe with the so called Nuclear bomb, since now we know there is a Company manufactuaring Gantz balls, there could be another location where this same Company is manufactuaring Nuclear Bombs of that same quantity.
Also back in the 1st stage, when Kei meet Nishi, and he made up the story about how all the politics have no idea what is going on and they are just puppets under a hidden Organisation that hunt down aliens and recruit people through Gantz ball. I feel he might be telling "Half-truths" i.e lies about himself being the son of an important politician, getting money as rewards and other nonsense to lure the newbs as bait and evaluate the 'Target's Strength'. Once an incident reaches a certain point, the 'shadow' government will launch a nuclear strike and thats the beginning of Katastrophe, seems Nishi is still talking about Politics and behaves like a Bureaucrat. Not to mention, whenever he talk, he is usually correct, logical, Meaningful words that ring inside peoples mind but does not get taken seriously by it, he gives out useful information and hints but they do not take him seriously this might lead him thinking there is no point telling it to someone who will probably die so he lacks going further in his debriefing and seems to have connections to other Gantz players throughout the world by his website he is able to access (not his journal, it was mentioned that he has access to websites none of the other Gantz players hes with know of, not only that but he had a brief discussion with Izumi about requesting help from other countries or let them handle the situation) Just a Thought and I could be wrong since GantZ has the most unpridictable storylines of all manga.
Most of his actions are actaully based on a whim, he alway gives a certain look and grins after revealing a little shred of information, with his nature he doesnt need to tell those Gantz players Anything and leave them all clueless to die but he does so anyway, carefully not revealing too much. He has died from Tanaka mission because he went on a whim to be the first to kill instead of using is natual stealthy tactic, He lost an arm from Nurarihyon mission being reckless and not waiting enough instead he decides to try a hit.
Who is this stranger you mean that he is going to spill the beans for, I didnt say anything about him telling anything, I just want to see the reaction of those faces he killed appear infront of him again with their second life. He is actaully Mature for a mere 14 year old but lost his sanity from no where to go back in his real life, and was bullied so he has now become a sort of bully himself by reacting the same way, the only place where he is really wanted is from Gantz, who only wants him to kill, so how can he not kill when the only thing he knows in his life is killing.Yes I agree that he is selfish because he looks after on his own and is alone but how is he a jackass? because he killed "kill off characters"?