You know, I'm really glad I gave this manga another chance.
When I first read it (up to the part where Kurono's girlfriend becomes the target), all I understood was that they get transferred back to Gantz again and again to kill aliens Gantz had created. Don't worry. I now know Gantz didn't create them. I didn't like Kurono at all because of his behavior in the beginning.
The artwork took me a little while to get used to. The first few girls that Kurono goes through are very typical big-breasted women who are obviously attracted to him because of his heroism and his confidence to fight (almost without fear). Not to mention that at the start of every chapter, there's always a pinup of some naked or half-naked chick with some cool weapon or vehicle to show off her body. Just some shitty ecchi to keep audience invested. It gets old fast. I've even considered stopping because seeing so much it made me assumed that maybe this was just gonna be a shallow fighting manga. But I'm glad I stayed with it. Despite all the naked chicks, the artistic ability of the characters (later on in the manga), the "aliens", the fighting scenes and so forth were extremely detailed.
The story has great character development. I have to say that the buildup of the Gantz killings to Katastrophe was pretty good; each Gantz game makes a little more sense as to the purpose of it. The Vampire Arc, however, I find it utterly boring and irrelevant. This was where I find the need to question the mangaka as to why he plopped it in there. I think this Vampire Arc with Kurono's brother was introduced so that we can further dwell into Kurono's past, but it was poorly executed. And what about his brother (who was a vampire), as Shinjite Florana stated, seemed like he might've played a pivotal role but then was killed off screen? Well, that was a waste of time to get to know his brother then. Two vampires joined them in one of their Gantz adventure and when it was over, was never seen again, almost as if the vampire arc never existed. Then we have the ending, Katastrophe, after Kurono defeated an Alien war veteran, drops back to earth, and then...end. I wish more could have been added to it, like Kurono's true happiness.
This is one manga I'll never forget. It had all the components to a good manga, even though I've stated quite a few wrongs in it. I definitely recommend to anyone who likes dark action, tragedy, love (not boring romance), and heroism (with a little ecchi, I guess). Enjoy your reading and hope this helps.