Most fans were genuinely happy with how Gantz:0 turned out. It brought one of the best arcs in the whole series to life with some of the most beautiful looking CGI money can buy. The fight scenes were a joy to look at, especially towards the end of the movie. The battle between Oka and Nurarihyon was as intense as it was in the manga.
Although the action and animation were top-notch, it did change quite a bit of the story around. Kato was still the focus, but many things were shifted to help create a more contained narrative.
10 Absence Of Characters
There's both a good and bad to this decision. On the one hand, leaving out Kyo Hanaki and Kazuo Kuwabara was a smart decision. It helped rid the arc of two wholly unlikable characters as well, letting the focus center more on ones fans want to pull for. The lousy portion is that the Tokyo team was cut down immensely, whittling it down to only five characters rather than the more robust cast.
9 Kato's Lack Of Experience
The film portrays Kato as almost a complete novice, having no idea what's happening in the game at all. It's a little understandable when fans consider the movie needs an outsider to help those unfamiliar with the universe. He's there so everything can be explained to the viewer. The anime tries to explain some of that with the ending.
8 Kato's Mind Being Wiped
It's in service of a twist at the end of the movie and didn't come during the Osaka arc of the manga. Kato being revealed to have been a former participant in the games who escaped just felt unnecessary and a twist for the sake of having one.
It didn't add all that much to the storyline as Kato was far from some hidden powerhouse whose memories were crucial for defeating the final demon. It's one of those changes films make to put their flair on it.
7 Kato's Motivation During The Arc
During the arc, Kato's main goal always revolved around reviving Kei Kurono, his best friend and the main character of the entire series. He even goes seeking out the 100 point demon because of this motivation. The movie wipes all of that away as Kei has hardly any role in it at all. It's somewhat understandable since Kei isn't in the arc the film's based on, but it still undercuts Kato's character and forces the massive changes in the movie.
6 Anzu Getting Revived Instead Of Kei
Since Kei plays a minimal role in the film, he's naturally not the one who's brought back by Kato. Instead, it's Anzu who developed a bond with Kato throughout the movie and made more logical sense with the story they were trying to tell. Outside of the action, their connection was the best part of the animation. In the actual manga, Anzu isn't revived by Kato at all and is instead brought back by Tsuneo Nikaido, a player who idolized Kato.
5 Ending Of The Oni Arc
The oni arc precedes this film and is shown in the opening scenes to give a little backstory. It also happens to be the only time fans see Kei since rather than surviving the mission, as he does in the manga, he saves Reika and fights off the final monster by himself.
It's also how he dies, skipping over the entire vampire storyline, which isn't a bad thing as it was one of the sillier things the manga added to the series. Given how great the scene was animated, killing Kei off in this way can be forgiven.
4 Excluding The Vampires
They were nowhere to be found in the movie, and it made the story easier to follow because of it. Frankly, the vampires only muddled up a story that was at it's best when it was simple. Having them kill Kei and their involvement in the arc just felt superfluous, especially when fans consider they play a small role in the alien invasion that follows. For as cool as Hikawa or Host Samurai was, the movie made the right call in cutting them.
3 Characterization Of Side Characters
While Kato went through the wringer as far as how much his character's motivations and story were changed, he was the main character of the movie. He ended up receiving the brunt of the development, leaving many of the other characters out in the cold as completely one-dimensional cutouts. It's a shame considering how cool guys like Nishi could be when utilized right. In the end, they were all just there to service Kato's storyline.
2 Nishi Murdering A Team Member
There is no doubting that Nishi is far from a team player, often going off solo during a mission and thinking himself the strongest of all of the members present. He's prone to talking down to others and being a genuinely surly individual. The movie decided to double down on that fact, making him homicidal as well. He kills an unnamed team member at the start of the film, saying that he would have been a liability had they lived. It was a cold-blooded act that his manga counterpart didn't do.
1 Nurarihyon's Defeat
Much of the final battle is the same, from how Oka makes his impressive entrance to their intense fight to Kato devising a plan of bringing the demon down. Even Anzu's death is brought forth similarly. The big difference is the absence of Hikawa, who bisects the devil from behind, allowing Kato to blast it to death with his specialized gun. While it is different, the way the movie handled the fight was equally good, showing what a team effort it took.