All of the sexy pinups that appear in Kurono Kei's bedroom are actually cover pages from the Gantz manga.
The Japanese TV broadcast of season one of this series was heavily censored to remove the graphic violence and nudity that it otherwise possesses. However, the DVD releases, as well as ADV's release, are complete and uncensored.
In the long version of the opening, you can hear samples from the Atari 8-bit game Archon.
The original TV version of Gantz was heavily censored. And besides the usual editing episode 2 and 3 got cut together and episode 10 did not air at all. The basic structure was left intact.
Based on a Japanese manga series written and illustrated by Hiroya Oku. It was serialized in Shueisha's seinen manga magazine Weekly Young Jump from June 2000 to June 2013, with its chapters collected in 37 tankobon volumes.