I just binged on both seasons of Gantz and I thought it was hands-down excellent. While the action scenes were admittedly terrible (someone throws acid at you, let's have a monologue while the acid flies through the air in slow motion but don't get out of the way) I was really drawn in by the extreme cynicism and grittiness that went beyond anything I'd seen in anime before.
One of the things that I found to be an incredible accomplishment was that I finished both seasons having some form of dislike for literally every single character that appeared, whereas other series created a sense of cynicism by targeting an innocent character (Speed Grapher) or instead tried to make gray characters likeable (Black Lagoon).
I have a feeling that the manga for Gantz will omit some of the cheesiness from the anime, but I'm also looking for something else that has a similar amount of crapsack world in it. Any recommendations?