The ending was horrid.
It tried to explain Gantz as something personal, related to Kurono, but it failed.
The final scene was a pure Bebop-ripoff.
The last episode was like one long slap to the face. Probably the most insulting moment was when the idiot writers decided to politicize Gantz. I almost stopped the episode right then and there. Politics in entertainment is fine, as long as it makes sense in context. Using Gantz as an outlet for Bush-bashing, though? What the hell?!?
The final arc was terrible and nonsensical. Gantz had no real reason to want Kurono dead. Even if he DID, he could simply detonate the head-bomb. There was no reason for what Gantz did. It was little more than a glorified McGuffin.
The characters...blech. They were not interesting. The manga went the proper route after the Buddha mission. Furthermore, Kurono's behavior didn't make sense. The Kurono we knew after the Buddha mission would have realized saving the two psychos would be impossible. He would have blown them away, and we know he could have.
We know full well that bullets don't do jack shit to Gantz suits. The bit was proven long ago.
In short, Gantz's final arc and ending sucked. Every time I tell people to watch Gantz, I tell them to skip the last arc and read the manga instead. They're better served that way.