So, a star system was ending. Various races started migrating to other planets. Some came to Earth and ended up fighting the Gantz teams. The 'end' or 'catastrophe' was referring to the date the giant aliens were due to arrive, who planned to conquer the Earth. Gantz was gifted to the Earth by an even more advanced alien race to combat the giant aliens, but the recipient turned it into a sort of game. I got all that, but the logic and reasoning of the truth alien seems babbling and inconsistent. I also don't understand why the 'God' theme is so present in this manga when there are aliens and monsters running about, not to mention people being constantly revived. What's the commentary? Why would any of the characters believe in God? It's difficult to work out what the author is trying to say.
