I should clarify that i'm not necessarily offended by violence or sex; i'm a huge fan of other violent seinen such as Berserk and Vinland Saga. However, the author's main goal seems to have been to shock the audience as much as possible in order to give the illusion of quality.
In Gantz, people who die are faxed into a room by a black ball that hands out weapons, talks in 'leetspeak', and tells them to kill aliens. That is literally all the story you need to know or will be told.
The first 100 or so chapters of Gantz are nothing less than loathsome, and this begins with the characters. The main character Kurano is an unlikable douchenozzle, and that's his only character trait (besides his misogyny, which we'll get to); his friend Masaru is equally shallow but is instead a 'white knight' to contrast with Kurano. The worst example is Kei, whose appearance is as a naked suicide victim and is shamelessly exploited by both the author and by the other characters. Kei is seems more like the author's blow up-doll than an actual character; she is constantly being molested or threatened with rape, and each of the early chapters is preceded by an official drawing of her in an explicit pose.
Characters don't have to be deep for a series to be good, but Gantz goes out of its way to make each character as shallow and unlikable as possible. Most of the minor characters that come to the Gantz room are gang rapists, and the main characters aren't much better. Somewhere around chapter 120 the main cast becomes somewhat interesting and likable, but at that point the series is caught in a vicious cycle between being really dumb and really offensive. The only character with a real arc is Kurano, and it can be summarized by 'got a girlfriend and became slightly less despicable'.
By far the worst part of this series for me was the way the female characters are treated. Even after the 100 chapters of exploiting Kei have ended, each female character introduced after that is completely defined by their crushes or codependency on one of the male characters, who treat them like garbage. This trend only gets worse as the series goes on.
Almost as bad is the relentless, naive cynicism at play. Every person who isn't a main character is portrayed as mean, stupid, and pointlessly cruel, especially the teachers (gives some insight into the author's life, possibly). Horrible things happen in this series for no reason other than to shock the reader. When a mass shooting occurs, it's because the author wanted to draw a bunch of people being machine gunned to death, not for any narrative reason (it is barely mentioned later on). In short, the world of Gantz is one that is frankly not worth saving, and this becomes tiresome to read.
The sole high point of Gantz are some of the intriguingly weird alien designs, especially early on, and the level that the art gets to later on. I personally found the art to be uninteresting, but you can't complain about its overall quality.
Against all of these criticisms, the other nitpicks i have seem irrelevant. The lack of any narrative flow, the poor pacing of each chapter, and the out=of-place story elements (there are vampires and psychics introduced that have nothing to do with the aliens, without any explanation at all).
If you want to read violent schlock, go read Hellsing (which is REALLY GOOD violent schlock). If what i just described sounds like your cup of tea, then god help you. If you like story, character, or investment on the part of the author, then find something else to read.
