The real question should be, "why didn't Kirk just grab the phaser rifle when Gary was normalized and blow him away instead of having a brawl?" I mean, aside from "the episode had to end with a fistfight in a ditch."
Kirk knew he didn't have much time. Mitchell is not just going to stand there while Kirk runs over and gets the phaser rifle. He most likely would have used the rocks as cover; causing Kirk to search around for a clear shot. He obviously knew he didn't have time for that and thought that he would get GM while GM was still on the ground.