I’ve not yet watched any Gantz but I know two crucial things about it; it’s supposed to be very, very violent and the anime covers feature a mysterious black orb.
This video is clearly Gantz. There’s no violence but that black sphere just screams “Gantz” at me and I suspect that would make the director very happy. The video itself contains no spoilers. Perhaps they couldn’t find more than 15 seconds of violence free footage?