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.
There are times when one of my favorite manga's is turned into an anime, and I either get excited or I want to punch a baby (not really, but you get the point). Gantz got me excited, then I reallized that I was sitting through some of the worst filler I have ever come across in a while.
It goes into pointless plotlines that have absolutely nothing to do with the story, and Gantz will spend nearly entire episodes on how an old woman that was barely mentioned in the manga has a major role in the show. Its a cop out for people who read the manga, and it will piss you off. Not to mention that the manga is still ongoing and the anime barely covers the three missions out of nine or so that happen in the manga.
If you are still reading this then, I have to say GO READ THE MANGA, unless you love filler that much.