To reward people for posting is not a bad idea. But you can do that without giving the characters gantz points although the didn't actually kill aliens.
You kill aliens and get Gantz points until you reach 100 and get....a stronger weapon (nobody is gonna go free right?).
But the other players that run around and did not get kills, gain experience nevertheless. The learn how to handle the suit and the guns better.
And that's the reward you should give them. Simple exp points.
And people that kill aliens get exp points and gantz points.
Any other system just seems wrong. If someone fights the alien head on in close combat while another snipers the alien and kills it, both would get points in your system. Which is nice of you, but not very Gantz like.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"