I watched the anime and read the manga. But really I don't see what the fuss is all about. The premise of the story may of been new when this initially came out, but nowadays there are better variants of this survival game genre that are better written.
For one thing I dislike about the manga is amount of gratuitous nudity that's in it. Don't get me wrong I like boobs as much as the next guy, but way it's placed it feels cheap and unnecessary. For example, almost every chapter cover page has some chick with her boobs hanging out (that have nothing to do with the plot). The author even places these between dramatic events. Like say what is suppose to be a shocking grief stricken scene where one of the protagonist's teammate dies, you go to the next chapter transition, and suddenly you're staring at a pair of knockers... It's more disruptive than entertaining to say the least. Then there's this obnoxious vibe that's present in the manga that feels like, I don't know, "exploitative" to women? I found the whole manga filled with too many cruel and obnoxious people like rapists and thugs to be enjoyable. Add this to amount of gratuitous nudity that's in it. It just gives the whole thing a bad feeling that I'm not a fan of.
But the real down point of the manga is not that, it's how poorly written and how frustratingly dumb the characters in this story are. Take the protagonists for example, the story begins with them dying because they tried to outrun a train instead of stepping back onto the platform to avoid it. What kind of an idiot does that? I think the excuse was that they forgot it was an express train and thought it would stop by itself. But if you stop for 2 seconds, you'd realise how stupid that is. One, they had been waiting on the platform in the first place so obviously they would know what the next train would be. Two, unless they were waiting at the very end of the platform why would they think it would be faster to sprint to the end of a long train station platform instead immediately getting back where they jumped off?
The whole manga is filled with poorly thought out writing like that. You got civilians constantly getting themselves getting themselves killed because they stand around foolishly, even taking photos on their mobiles instead of immediately running away from danger. The way they nonchantedly spout lines like "Is this CGI" or "Oh they're shooting a movie" before dying just beggars belief.
Another example would be the Gantz players. They are given these unlimited ammo sci-fi energy guns that are "accurate to 1km". These godlike wpns can even "lock on" and not require line of sight. As well as super suits that can jump buildings and guarantee mobility. The enemies they're asked to fight are basically nearly all melee range aliens. So do these idiots do?
They run right up to the alien just to "shoot" and get surprised when their hands get chopped up. To top it all off, they like to make faces at the aliens for a long time before they actually pull the trigger. There is literally ONE guy in whole manga that realises what they're holding in their hands are in fact GUNS not baseball bats and shot the aliens at long range. But inexplicably the guy decides to run close to melee them half way through and inevitably get himself killed.
Finally there's that stupid god awful ending. Without spoiling too much, Earth ends up almost destroyed bc after the defeating the aliens, the Gantz players were too busy raping alien prisoners and forcing them into fights before properly securing the mothership. I'm not kidding.
To this all I can say is WTF Japan, WTF.