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.
Mix 'em up with Matrix, where you will see politics, blood, nudity and not so kind main male character. And when Gantz make appear a character it is so detailed.