When I have cooked a turkey on a gas grill, I used the same temps and times as I would if I cooked it in the oven. I season the bird and coat with melted butter. Put it in a drip pan, I also use a small rack to hold the bird off the bottom of the drip pan. I use higher heat at first to brown it and baste it with butter several times while browning. Then I turn the heat down and tent the bird with foil. Your plan to use the outside burners only is correct. I did turn on the middle burner on it's lowest setting for awhile also to speed up the process. Good Luck!! It will turn out great.