I have a Gantz theory that explains near EVERYTHING
Now before you chew me out for thinking I'm some know-it-all newbie. I'll give a little bit of info on my self. I've been lurking these forums for the past 3 years, and the manga was my only source of entertainment in my first months after purchasing a home. I'm pretty sure I've read every episode's thread and think you guys are awesome. I don't think I'm awake enough to write a multipage theory but I'll try to make it thorough enough to catch my drift. If you have any "What about X?" questions, I think I can answer them.
If you don't like long stories. Read the parts in bold as Cliffs Notes
Okay, here we go - Chronological Order
Several Hundred years ago , there are several races living in semi-harmony throughout the galaxy. As highly advanced aliens are prone to do, they start travelling from planet to planet. At some point, a relatively small number of aliens come to earth (via teleport, I assume) to live a fresh new life in disguise and multiply. After a few mishaps in keeping their presence undetected, (ex. roswell) Earth becomes aware of the aliens' existence not as spaceship invaders but long-time inhabitants with questionable intents. The aliens at this point just want to live out their lives unnoticed.
To counter this foreign presence, Humans develop nano technology to create vampires and fight off the aliens. At first the vampires are well supplied with weapons, money, organization, and technology to aid in spotting the aliens (ex. their sunglasses). I don't know how vampire-to-bes were chosen. My guess would be a lottery in which the government picks out and injects the nanos into people who have a good fighting ability (similar to Gantz recruitment without the deaths). The battle between aliens and vampires rage on under the cover of darkness for several decades until Humanity makes a new discovery - The Akashic record.
Quoting wikipedia, the Akashic records "are described to contain all knowledge of human experience and the history of the cosmos". This girl who accidentally patches into this goldmine of knowledge then begins to spill the beans on the not-on-Earth Aliens' technology which would also be contained in this record. The Gantz technology would replace the vampires in the war against the interstellar immigrants. The old man in Germany quickly spreads the Black Ball technology to several countries so that they can each exterminate the aliens one by one.
From here on, I'm going to refer to the on-earth aliens as "immigrants". "Aliens" from here on refers to the guys who never left their alien homeplanet(s) and got to play with all the cool technologies for the past hundreds of years. The no-longer-needed Vampire organization is cut off from the government and left to live in the streets and live less-than-glorious lives as their numbers dwindle. It is at this point that the Vampires set their sights on the Gantz participants as revenge. The existence of Gantz now threatens the existence of both Vampires and the immigrants - leading to an uneasy alliance. The Vampires are much fewer in number, so their ability to combat Gantz is limited. Now heres the plot twist:
Sebastian catches word of Humans copying Alien technology from the Akashic record and decides to wipe them out before they become too strong. The first step was to gain influence over the source of information (the german girl and her father's company). Second step was to deal with the 2nd largest threat: the millions(?) of long-term immigrants (who are potentially very powerful) who've gotten cozy on Earth. In reality though, Sebastian and the true aliens could kill the immigrants easily. He's letting the Gantzers fight because apparently the Germans want to gamble on who the better fighter is. (In his travels, he for some reason teaches Sakata an ability that is yet to be revealed by the the german girl. Perhaps he was to be a human vessel in case things went wrong)
Under the supervision of Sebastian, the technology of Gantz was modified a bit. The Y gun would send the immigrants to Sebastian's planet/ship to be punished/executed/interrogated, he'd include a time limit & rewards so that the Gantzers are too rushed to ask the aliens & immigrants whats really going on, and he put in a monitoring device that would kill any tattle-tellers and keep the weak Human population out of the loop. (eventually he turns it off because "Fuck it! Everyone knows now anyway"). As the attacks against he immigrants continue, the immigrants start rising up and showing their muscle. Sabastian realizes that eventually the immigrants will overpower the tiny gantzers and their tiny weapons. The countdown to "Fuck it. Kill them all" aka Katastrophe begins. The Roman 'aliens' had nothing to do with Katastrophe. They were immigrants who were so pissed that they were getting in larger numbers and starting to make serious damage against the gantzers. Coincidentally this comes right as the countdown hits zero (hence the early return teleport). The balls are turned off because it's invasion-time!
Katastophe begins when Sebastian's army comes down to punish humans for exploiting technology stolen from the aliens. I don't know if the immigrants are on Sebastian's side or not but I think they'll get killed in the battle just because they're a potential threat to the true aliens. This is why the devil-looking alien talks of a God. They're talking about the aliens of their home planet who have the power of Gods and can kill any one of them without a problem (Sebastian's psychic powers being only an example). The immigrants are really some low-tech aliens who've been out of the loop for several hundred years - thats why they live in apartments and not space ships (Onion Alien) and no longer wield guns. To the immigrants, the invading aliens are gods above, comparatively speaking.
Sadly with all this said I still don't know how this series will end. This is a pure guess but I think A) Everyone will die or B) The gantzers will Y-gun themselves onto the mother-ship/planet and kick ass or C) Uncover a doomsday weapon inside the big black ball in Germany (remember that?) and use it on Sebastian/the mother-ship/planet.
Okay, tear my theory apart guys! I'm looking forward to it :D