Overall Gantz is a good series that had potential to be great. The concept behind it is very interesting and it raises good questions about the human condition such as what a man alone can do against the concensus or how to handle your sexuality when it differs from the norm (even if at times in this show, the weird fetishes are portrayed as "normal"). Gantz falls short of being a better series as it loses some of its intensity and creativity towards the early middle.
The way Gantz really loses steam is through dragging out a story that could be resolved in 2 episodes with useless flashbacks and melodrama. Most if not all secondary characters are given a background story and the saddest it is, the more likely they are to die. You come to this conclusion while watching the anime as it tends to repeat its own cliches.
The background stories of the cast are dragged out so much you wish they had edited out a lot of what was seen, this way one would have cared about the characters had they known less of the unnecessary aspects of their life. That would also have helped them be more cohesive characters because as it is, their background clashes with the way they act in the present time - where the story takes place.
The animation is excellent in parts (I am a fan of the character design, Kei-chan looks really sexy in his suit. Some of the action scenes also have good detail) and not so great in others. The main reason being that some of the background are in 3D and the characters are in 2D, leading to some unconvincing melange. The frame rate is good in action scenes, yet very bad at other times (though one would assume panning to a character's face while he/she is thinking for 3 minutes might be effective, I think it might have worked better if their expressions changed a little more in that time frame.)
The sound is alright overall. The opening song is one frantic and plays up the action vibe, and the song which lets you know that the mission starts is also pleasant and efficient. These are the only great songs though as the series is then let down by some average tracks, including the closing song and the background tunes. Those instrumental numbers do their job but they are not the kind of thing one would want to listen to in great repetition.
Seiyuus in this do a good job but as the characters are dreary and poorly written, there is not much they can do to enhance their personalities. Kishimoto has an adequately annoying and detached voice that captures her character well. Katou and Kei sound adequate but cliched (in a shounen sense, whereby the feelings portrayed are straight out of flat performances). Overall, it was an efficient but unmemorable performace all around.
The story is an interesting one about what happens to a group of humans after they "die" and remains so throughout (unfortunately the story stalls from there). Suspense is built well in the early stages, however pacing issues got in the way of getting the story anywhere worthwhile. Some character cliches also drag the story down a notch. Had they made more interesting characters - or at least ones you cared about, the effect of the story would have been a lot stronger, since this is meant to be viewed as a character-analysis show.
I enjoyed this anime when I was watching it but it is unlikely to appeal to most people because of the excessive violence. The story is strong in parts but the repeated footage that does not achieve much drags down the effect a fair bit. Add the insipid character development and you get yourself a disappointing anime; not bad just disappointing.