I liked Gantz, but it really is just mindless killing for the most part.
The story is also a nonsensical mess. I think Attack on Titan is much better at trying to tell a cohesive story.
Still, I wouldn't complain if there was a Gantz anime.

There are several live-action Gantz movies. I haven't seen any though so I can't say whether they're any good (I doubt it).

As for why it isn't an anime - there is a ton of extreme violence and some nudity. Stuff gets censored in JoJo, so I can't imagine how much they would try to censor Gantz.
Additionally, these days, anime are often made to try to increase sales of ongoing manga/LN. As Gantz is over, that isn't going to help as much.