The anime centers around a small group of, usually hovering around three, protagonists who seem to have died but are transported into an apartment that is, except for a large black sphere named Gantz in the center of the room, all but empty. They cannot escape the apartment. They are given a form-fitting suit and a variety of unearthly weapons. The sphere, through crude text messages, explains to them a target that they are to hunt. They are then transported into Tokyo city and left to their own devices. Some of them die, some of them live. Those that live are allowed to return to their normal lives for a time, only to eventually be transported back into the room for another round.
Gantz(the anime) utilizes the same style as the manga, melding traditional drawings with computer generated images. While it gradually loses its quality as the anime continues, this unique style begins as something fun to watch. You will be treated to camera pans through a computer-generated subway station, or the computer-generated Gantz room, with traditional cel-characters on top. While hardly seamless, it is different and interesting to see.
The characters are interesting, have flaws, and it becomes easy to care about them. The rampant, pubescent libido of Kei Kurono, and the exchanges between him and Kei Kishimoto's breasts, is hilarious and offers a much needed levity to the seriousness of all the violence and mayhem.
Speaking of violence and mayhem. It is worth mentioning the violence, language, and sexual content of the anime. It is often gratuitous and visceral. If you are put off by this, then Gantz may not be for you. On the other hand, this may be what has lead you to Gantz.
While the series starts off very well, it consistently has very SERIOUS pacing problems; especially during scenes of action or gravitas. There are episodes where a whole half is spent on mere exposition about how crazy and wrong everything that's happening is; all while guns are drawn and a monster is bearing down on them. This may sound exciting, were it not for the amount of time the characters just stand there with guns drawn.... talking. I suspect this is because the anime is largely faithful to the manga; almost word-for-word-frame-for-frame, and this is its problem. While it's o.k. for a character to have an internal monologue in a book, it is excruciatingly tedious in anime to have the action come to a standstill so the characters can converse with each other and themselves over the morality of the situation. I am a firm believer that anime(and movies and television) should SHOW, not tell. There's no need to break into soliloquies, or to outright tell me how crazy and wrong everything is. Also, some of the things the characters say, in the moments they say them, is incredibly unrealistic and clearly comes off as long-winded character exposition.
The series created an ending that swerves from the manga's storyline, since the manga did not intend to finish and was still publishing(the anime had also caught up to the manga storyline and passed it in the final volume). So they created an ending that is completely unsatisfactory, un-climatic and so ludicrously open-ended that it can hardly be called an ending. You could hardly believe it was the final episode if it didn't say so on the DVD case.
Also, there is a SEVERE drop in quality nearing the end of the series; especially the final volume. So much so that the show can hardly be called "animated" and is more a series of still drawings with voice-over. There is an episode, during an action scene, where the anime literally shows us still images with absolutely no animation, making very obvious some kind of cost-cutting measures, perhaps because of deadlines or laziness. It looks bad.
All said, Gantz begins very well, gets a little too slow at some point along the way, and then flat-out crumbles towards the end. I would regard the Gantz anime as a must-see, although you would be forgiven should you give up mid-way through the series.