I'm writing this from the perspective of someone who has never read the manga, watched the animes or played the game. Gantz isn't 'bad' but it's not great. It's pretty unique and well-made, but it doesn't really GO anywhere. IMO, the plot does not advance enough. Without spoilers, the plot is basically: be summoned, battle an enemy, people die, those who survive go back, and then are summoned, battle another enemy, etc (the process repeats 3x). I wouldn't call this a fatal flaw, though, since this might be what some people are looking for, and the sci-fi elements are pretty cool and the aliens are different from what you see in western media. I just feel like it was very repetitive and a lack of deeper character development meant that when characters were killed I wasn't particularly bothered by it. More specifically, I just finished Alice in Borderland, which properly developed characters and therefore deaths were extremely painful and gut-wrenching to watch, so by comparison Gantz fell short here IMO. Still, Gantz isn't 'bad' in this respect.

However, when it comes to the presentation of female characters, Gantz is god-awful.

The focus on the MC's perverted desires at the start & the way he treats women is uncomfortable, but that wasn't the worst part. The presentation of female characters is just the epitome of 'male-gaze'. That is the one thing in this movie that I absolutely hated, and meant it had very little re-watch value to me. Female characters in this movie only existed for the men, only really talked to or about the men, and only existed to further the character development/storylines of the men. Definitely would not pass the Mako-Mori test in any way, shape, or form haha. It's a pity that a promising sci-fi movie (/movie series) couldn't develop the characters properly. If they were going to make the presentation of female characters so offensively bad they might as well have just made it a male-only cast. If you can get over that and are fine with female characters being mostly just delicate and tearful and/or only existing to advance the plotlines of the male main characters (eg. profess their love for them, rely on them, save them, do/say X to improve their self-worth), then I would say give it a go. It's not great but it's interesting sci-fi