Here's a method for baking pizza on a gas grill.
Set a pizza stone on one side of the grill, place bricks on the near and far edge of the pizza stone.
Place a baking sheet on top of the pizza stone, and angle another baking sheet on the other side of the grill. Optionally cover the baking sheets with foil if you think they might get dirty. (Mine didn't seem to.) Use another piece of foil to cover the exposed side.
The idea here is to gather heat from one side of the grill and direct it over the top of the pizza like a pizza oven. Turn your center and right burners on high, and the burner under the pizza stone on low. Leave your grill lid open. Preheat the stone.
Remove the left baking sheet and front brick(s) to slide on the pizza. Make sure you rotate the pizza every 90-120 seconds or else your crust will burn like mine did.
Not the best cooking method, but if you don't want to buy a pizza oven it's worth experimenting with.