I have to disagree, with the "rules" of the game, they nearly match. Both can't tell others about it, there are secrets behind the "travels", point system (one counts up, the other down), and so on.
Not only that, but the theme to survive in hopeless situation, in total isolation where no one can help you. Then the "importance of team mates" part comes in. The only real difference is one is trying to save the present, while the other is saving the future (in a way). One uses technology as weapons and transportation, the other uses PSI. And unlike Psyren, we have yet to know the truth behind Gantz, but I am pretty sure it's similar, since Gantz, like the Nemisis (spelling?) Q, seems to know the future of the world (*
spoiler: Gantz's timer as shown in the recent chapters is counting down towards a future event*) and both are trying to save it by sending people there.
Really, the concept is nearly identical, the only difference so far is that we have yet to know Gantz's objective, which I am sure is similar to N.Q.'s (saving the world, but in a twisted way of sending only a few just so they die). One is against high tech ALIENS in weird shapes, while the other is against strong PSI HUMANS and alien looking creatures. If you take those away, they are really the same.
Well since this isn't a Psyren discussion (it's hard to post their similarity without spoiling both Gantz and Psyren @@), I will end it here. Have a good weekend XD (time to sleep, too many midterms, barely getting any sleep, sorry about the grammar, will fix it afterward)
And ain't you from that livejounral website? I visit it all the time
To check out new Jump manga and the ranking (But I rarely post as I don't have an account)