^ yeah it is kind of messed up at times. When I first started reading gantz I had trouble following it too. I thought of it as a "fighting manga". When I realized it's really more like a drama/mystery it started making sense.
Here's what I think about the story, and what it's underlining themes are. I think it's better to find out on your own, but I'll put this here just in case. It doesn't contain significant spoilers but I'll put it in spoiler tag anyway.