Character development Kei went from an asshole to a pretty decent guy throughout the manga and it's amazing comparing the first few issues to the last few on how much he's matured. You can also see this in Gigant but this is a Gantz subreddit so I'll focus on that.
Writing realistic characters/ the duality of man I liked Reika, Clone Kei, Samurai Vampire, Kaze, Cherry/Cherry's sensei, and so many others.
Two things that need work
I think that there's a lot of unnecessary filler in the final fights that should have been avoided- there's a lot better story telling than "shitposters on a 4chan forum" that can be used to give commentary, and "people around the world cheer", and tae crying for what is (by my count) 20+ pages in the last 10 issues of the manga- you could have had a wrapup chapter with those panels!
This is minor but I think worldbuilding in Gantz could have been expanded far more- there's an American team and an Israeli team- why couldn't we learn more about them besides the final fight? Would have been really interesting to see the other teams.