Gantz tells the story of Kei Kurono and his friend Masaru Kato who die in a train accident and become part of a semi-posthumous "game" in which they and several other recently deceased people are forced to hunt down and kill aliens armed with a handful of futuristic items of equipment and weaponry.
If u didn't read the manga before it, u probably will enjoy it ... this is an usual action anime, with a real-like world of todays Earth, with an excessive amount of blood and violent death, and ofcoz with a lot of "ecchi" (sometimes going into the hentai category) scenes. A story is being present just to roll the fights on, nothing serious, but u get some glimpse thou from the motivation of the characters. It has a good opening and ending, fit to the anime, and the animation is also top-grade.
Interesting thing, and for me the most valuable in this anime, the changing of the protagonist's personality. This evolving is a bit slow and sometimes weird, but it's really a complex and deep one. I can't say more without spoilers.
If u read the manga ... well ... consider this nothing too serious, at least u see the characters in motion. ^^