The Gantz anime series is an amalgamation of events that occurred in the first few volumes of the manga. It's an introduction to the Gantz universe and its inhabitants--most notably, the aliens and their human hunters. Like the manga, the anime centers around the growth and emergence of Kurono (Chrono) as a hero and his effort to win and survive at all costs. There's a good deal of sex and violence as well as long stretches of philosophical banter and social critique, but it often comes across as being so self-righteous and ultimately defeatist that most messages would only be appealing to audiences that already had the same beliefs.
The artwork is dark and is true to Oru Hiroya's concepts down to the intricately detailed weapons and power suits. The characters range from the sexually eager and aggressive Kurono, to the gallant Katou, to the buxom Kishimoto. The animation is mostly satisfying. Since Hiroya's artwork uses 3d models for scale and proportion, they can seem stiff and often lack fluidity. That same flaw is evident in the anime as well.
The story is intriguing, at the least, and seems to be inspired by movies such as The Cube. The sci-fi aspects are cool and even I would like to run around in a power suit for a day or two.
Altogether, it has some great ingredients that are marred by plodding progression and preachy characters. The fan-service is there, though not to an excess, but this is certainly not for children or those that are easily offended by violence, gore, or opinions diametrically opposed to your own. It's definitely worth watching and depending on who you are, you might just like it a lot.