Gantz isn't really about the fighting. It's more about how the fighting/killing affects people and what they are willing to do to survive. It examines society in general and how people behave differently around strangers then they do around people they know. It also explores the dark parts of the human soul. At times some characters have to choose between their own well being and the others in their group. This brings up the notion of "how much is someone's life worth". To some it's priceless. To others it's worthless. Is someone you know worth more then someone you don't? Aside from the "Is one human more valuable then another." issue, there's the question of "Is a human life worth more then an alien?". The first 250 chapters revolve around this theme.

From 250 onwards it becomes more of a mystery. They start to question what exactly gantz is, and what purpose it serves. The fights/hunts become secondary (although cool as hell). They begin to find out more about gantz and it's origins.