I have been able to cook some awesome pizzas on my gas grill using a pizza stone and a heavy-duty wok cover. The work cover is 15 inches diameter and comes with a riveted aluminum handle so it won't burn (like the wooden plug on my flimsy wok cover from my old wok), total $32.96 (including shipping) at Restaurantsupply.com
I made the mistake at first of putting 2 pizza stones on the grill, but the stones would get super-hot because they completely covered the grates so there would be no flow of heat in the upper part of the grill for the toppings. The solution was to use a single stone in the center of the grill and the wok cover to keep the heat close to the top of the pizza.
I fire up the grill (outer burners on high, inner burners on low), and in 20 minutes the stone can get up to 650 degrees (I carefully monitor the stone temperature using an IR thermometer and decide if I want it at 550 deg., 650 deg., etc.; temps above 650 deg can result in scorched bottoms which wife doesn't like). I then turn off the inner burners, put the pizza on the stone then the wok cover over the pizza on the stone, then cover the grill lid. I turn the pizza around at 5 minutes for even cooking, and 5 minutes later I have the perfect pizza!
I can usually put on a second pizza right afterwards, maybe will put on the inner burners on low for a few minutes.
Much better than the lame 450 degrees on my home oven, and keeps the heat outside during the summer.
This is quickly turning into a wonderful weekly pizza adventure (first picture is the pie coming off the grill, second picture is after it's dressed with fresh basil)