This is a review for the entire series.
Gantz is an odd story. No doubt about it. It is heavy on nudity and gore, and makes me wonder if it really needed to be that way to get the story across.
The artwork and detail is extremely well done. Oku has an immense imagination and I spent a lot of time looking at the painstakingly created backgrounds in each panel. All the enemies and characters are unique, and there are many of them.
Gantz is so heavily focused on action, that is doesn't lend itself to much exposition or explanation of the story. By the end of the manga, I still had many questions, and there were many plot holes left. The ending was what I expected and I was fine with that.
There is some romance and emotional stories within the manga, but it's very light and more to shape the main character rather than to create side stories. I thought the females character were unrealistic. More like a teenage male's dream, physically and emotionally.
If you like bloody action with a large dose of boobs, I would highly recommend Gantz. If you're not really into action manga, then there is nothing redeeming in Gantz to make you like the story.