Gantz is a manga by Hiroya oku published from 2000 to 2013. It has been adapted in an anime of two season in 2004.
Summary of the anime Gantz
Gantz is the name of a big black ball which, regularly, invites recently dead people to play a mortal game. Such is the case for Kei Kuruno and Masaru Kato who, right before being crushed by a train, were transferred in an exitless room. In this room they would find people with similar stories, and Gantz. After a while, Gantz opens up, revealing the weapons and armors inside, to help them kill extra-terrestrial beings. Some questions remains : are they really dead ? and if not, will they ?
My opinion on the anime Gantz
If you forget for a second the sex and the violence, the anime was not that great. We get hanged up at the beginning with something that seems like an original story, then the anime gets repetitive and boring : “I kill another one, stronger than last time by outdoing myself again”. It could be considered a mature version of Pokemon in this way. Furthermore some scenes are protracting (like dragonball-Z protracting). Gantz is violent, with blood and body members everywhere, flirts with hentai-style sometimes (who is complaining, though ?) Some crude scenes raise the standard but some clichés lower it.
I heard the manga is not that bad, and if you are reading it I do no think I would recommend you seeing the anime. But if you have not read the manga and want to know what Gantz is all about, the anime is certainly an introduction worth trying.