The Making of Gantz (with spoilers):
Hiroya Oku first thought of Gantz's story when he was in high school. The inspiration of Gantz started from the Jidaigeki television program of Hissatsu series. He remarks being inspired by the Robert Sheckley's novel Time Murderer while developing the idea that dead people are transported to a place in which they are able to be revived. However, he still was not decided to make Gantz until writing the manga Zero One; Zero One had a similar setting to that of Gantz, but Oku ended the series, noting it was not very entertaining and that it was too expensive to develop.
When creating the chapters for the manga, Oku starts with a thumbnail of the pages. He then creates 3D models of the characters and backgrounds on his computer. Once done, Oku prints the characters and backgrounds he made in 3D, adds tone and color to the pages, and finishes with sound effects and dialogue. This style was already used in Zero One, but for that title, there was little work in hand drawing; Oku decided to add more hand drawing to give Gantz a more realistic tone as well as reduce the budget. However, he still notes that such a method is time-consuming and that he has to work quickly in order to finish the chapters on time.
Oku tries to incorporate realism into Gantz and adds that some of the events occurring in the story are based on his opinions regarding the world. During violent or erotic scenes, Oku makes sure to not make them very long to avoid reducing the series' realism. However, he has mentioned that he does not autocensor and that all the drawings he has ever illustrated have been published in the manga. Some plot twists are meant to go against common events that happen in several manga such as the deaths of the major characters like Kei Kishimoto and Masaru Kato. Before the series started serialization, Oku told his assistants that with Kurono's exception, all the major characters from the series would die.
The animated version of this series is actually an adaptation produced by Gonzo and directed by Ichiro Itano, aired in Japan on Fuji Television and AT-X. The Gantz anime is divided into two seasons: The first season is known as "The First Stage", while the second season is known as "The Second Stage", which is a direct continuation of the first season. The First Stage aired in Japan with several scenes censored due to inappropriate content such as violence or nudity. However, the DVDs from the series contained the scenes uncensored. The Second Stage aired on Japanese network AT-X on August 26, 2004. There are a total of 12 Japanese DVDs, released from August 28, 2004 to June 29, 2005. Additionally, the DVDs were compiled into box sets. ADV Films announced and licensed the series for release in the United States, of course in uncut form.
The biggest difference between the TV version and the DVD version folks, is that the "green onion" episodes did not air on television! So make sure you checked out the complete DVD version!!