Gantz has a purpose for everyone, read inside.
What is Gantz purpose and his goal? Well if we assume that there are other Gantz devices we can also assume that a war might be going on between these different Gantz. So the Gantz we know recruits people that die and sends them in mission so hey can train and fight Gantz war, this would be Gantz army. If we assume that this is a possibility then the 3 choices given by Gantz make perfect sense.
#1 reach 100 points and you are free and your memories erased. This is not entierly true thoug. We know that one of the people that reached 100 points got freed, yet Gantz gave him the black ball so he can come back.
#2 Get a powerfull gun. Why would Gantz have such an option to give a powerfull gun if everyone was to choose #1? Having this as an option is not really necessary then, unless it serves Gantz purpose.
#3 Revive a person from memory. This has double meaning, for one you get more experience as a soldier and two you get back another soldier for Gantz.
So whats in it for the Gantzers? Well, one its for sure, if they are good enough to get 100 points, then Gantz will never let them die, so living in a world where Gantz is your master would mean as long as you do good, you will live for ever!