If your serious about cooking the larger pizzas on the gas grill, here are my suggested modifications.
1) Find a friend or make a friend with a tile saw or masonry blade on a skil saw or grinder.
2) cut them to the round shape (red)
3) move the round circle of firebrick as far to the front as you can. (green)
4) put down a triple layer of foil around the front and sides of the firebrick (yellow)
Thusly, you are forcing the hot air under the stone, to the back, then up over the firebrick, out the front. The final element would be to add some heat deflectors (blue) to encourage the superhot, super convected air down toward the pizza. Carry on.