Gantz is about life, death, and the nature of reality. Let me start this review by saying that if you dislike R-rated content, and flinch at nudity this anime is not for you. It is graphic in the best sense of the word - violence and sex have a dark purpose in this anime, and they are omnipresent.
GANTZ is a looking-glass version of life itself. It is a game with obscure objectives and fatal consequences for failure. Information is pathetically limited, though the players have the tools they need. Those who can't adapt, die. Anyone who doesn't see a metaphor for the real world here is missing a large point.
Into this world come two unlikely heroes (unlikely in both background and temperment) - students who save a homeless man from a train at the cost of their lives. Throughout it we see their dark counterparts, two students who beat homeless people to death for fun. In the second season we see a twisted counterpart of the first, where the dark duo takes command of the group through intimidation and fear (as opposed to courage and charisma). The final evaluation on which version of life and leadership was superior? For those who have seen it, they know the fates of the group, and which fate was the better one.
For those who don't - watch GANTZ. Philosophy, action, suspense, interesting characters, and absolutely nothing resembling child-like morality and situations that make up the usual fare in TV. Nihilistic could be used to describe it, but the description does not give it justice. Worth watching, even if it won't cheer you up.