Gantz seems similiar to Zombie Hunter. Both Kurono
and Toshi-something are somewhat distant with
others and lost loved ones. Not only that but the
demon vampire
devilish things seem similiar to the "zombies"
in Zombie Hunter.

In case you didn't know zombie hunter is about
how a bunch of people were captured and sent
to an island (similiar to the "room") and then
were sent through several trials and became
stronger... yada yada yada dont wanna spoil it.