Another Gantz is a tv movie aired on Nippon Television Network right before the theatrical release of Gantz: The Perfect Answer and it's a kind of a link between the two movies.
After a trailer of the second part film, Another Gantz gives a summary of the first Gantz through the investigation made by a journalist who finds a strange text file on a file sharing site, relating about some people that are supposed to be dead and go on with their lives like nothing happened. It also add some new elements because as the journalist exchanges pictures and documents with the anonymous author of the file, the audience learn more about some characters and their lives after they met Gantz. So you'll see the reaction of the student Nishi Joichiro, a confused Kishimoto Kei tormented by the memories of her supposed suicide, and the familiar circumstances of Suzuki Yoshikazu.
The cast is the same as the first part film, with the exception of the "journalist" Kato Haruhiko. The extensive use in of the previous film scenes makes Another Gantz boring if you watch them at them one after the other. But if it's been a bit of time from Gantz, this tv movie could refresh your memory and prepare you to the following chapter. In addiction, Yamada Takayuki and Ayano Go play two misterious men following the journalist's investigation and apparently searching for "the perfect answer". This add a subplot that increase curiosity while you're waiting for the mistery to be disclosed.