Here is a recipe I found in my "Sunset Barbecue Cook Cook" I have not tried this particular recipe but I have barbecued a turkey in a very similar way but I can't find that recipe. It did turn out great.I may have to go to a second message as this is rather long.


BARBECUED TURKEY

20 to 22 lb thawed turkey (There's nothing to stop you using a smaller turkey)
2 teas. poultry seasoning
1/4 teas. pepper
1 cup port
1 lg onion, quartered
2 lg carrots, cut into chunks
2 stalks celery, cut into chunks
1 clove garlic, cut into chunks
3 or 4 fresh rosemary sprigs, each 3 to 4 in long

Remove turkey neck and giblets; reserve for other uses. Discard large lumps of fat, then rinse turkey inside and out and pat dry.
Combine poultry seasoning and pepper, sprinkle some in neck and body cavities and rub rest over skin.
Place turkey on it's breast and spoon 1 to 2 tbs of port into neck cavity, bring skin over opening and secure to back with a metal skewer. Turn turkey on it's back and place onions, carrots, garlic and celery in body cavity. Pour 1/3 cup of remaining port in body cavity. (it dosn't say but I would think you would skewer that cavity with metal skewers as well)
Place bird breast up on barbecue grill directly above drip pan. (will give the preparation of the barbecue either at the bottom or in next message) Cover barbecue and adjust dampers as necessary to maintain an even heat. Cook meat until a meat thermometer inserted in thickest part of thigh (not touching bone) registers 185 degrees or until meat near thighbone is no longer pink (cut to test). Total cooking time should be 4 to 4 1/2 hours. Allow at least 12 minutes per pound.
Several times during cooking, place a rosemary sprig on the coals to add fragrance as it smoulders. During the last hour of cooking, brush turkey with port several times, using all remaining port. When turkey is done, discard stuffing. Makes 16 to 18 servings. (It seems to me that the recipe I used had melted butter mixed in the port which I brushed on the turkey)

BARBECUING BY INDIRECT HEAT

Open or remove the lid from a covered barbecue; open the bottom dampers. Pile about 50 long burning briquets on the fire grate and light them. Let the briquets burn until hot (usually about 1/ hr). Using long handled tongs bank about half on each side of fire grate, then place a metal drip pan in center (aluminum)
Set the grill in place 4 to 6 inches above the pan. Lightly grease the grill. Set the food turkey directly above drip pan. Add 5 to 6 briquets to each side of fire grate at 30 to 40 minute intervals as needed to keep fire temperature constant.

I guess you would use a gas grill the same way. I think you could adjust the temperature better on one. I haven't done this for a number of years but I do know I did this over some gravel we had in the yard. It may be messy so if you do this on a patio or porch put something under the whole barbecue.