Gantz

I watched it all in 3 days, it had some good moments, but in the end there are still too many unsolved mysteries in the plot. So i want to ask everyone's opinon about it.
What (or who) is Gantz? is it the man inside the black ball ?
What's his purpose? just kill some people who just died ?
How are the selected people related to each other ?
And finally: do you think the hero dies in the end ? (i watched the last episode completely, and i'd say he does).
If you have any idea that would add some sense to this shaky series, please share your vision.
Thanks

What (or who) is Gantz? is it the man inside the black ball ?
This still question still don't have any answer.


How are the selected people related to each other ?
Who die when GANTZ want start the game . So each other don't have any related.

And finally: do you think the hero dies in the end
Yes .... Vampire kill him