Let me start this review by saying that I've never watched or read Gantz before in my life. I had seen some pictures around the net of the costumes (that look pretty cool, actually) but don't know anything about the universe.

In the Universe of Gantz: O some dead people get a second chance in the form of a "game" (with scores and all) they are divided in teams and called to missions, the missions are basically team deathmatches against various monsters heavily inspired by japanese folklore. After getting 100 points you can choose between 3 prizes: A better weapon/gear, to ressurect someone from your team or, to have your memory wiped and be able to live your life.

And in the movie you'll be following a day in the life of Kato, who after being killed on the way to his brothers birthday, gets his chance on the game. (he and his brothers are orphans, so without him the brother will probably be screwed)

The CG is pretty good, as well as the visuals of the main characters. You got your assortment of ass shots and overly giggly breasts as expected, but many soft-body physics on the monsters are very well made. There is a good diversity in design too, for the creatures! Many revolve around the "older stronger hairy man" kind of folklorical cliche, but they are each well made and have good animations.

The soundtrack is pretty forgetable and the dialogues are very confusing.

My main gripe with the movie would be that the rules of the game aren't made clear, I had a elevated tension level between almost all the movie because I misunderstood one of the rules. And some things are too fast/forced to sink in, like a romance between the main character and someone he just met.
There are some twists in the end and a cliffhanger, but mostly the plot is a linear battle of "courage against great adversity"

Good movie, short one too 1:30. Had fun but probably will forget about it soon.