Start this process the night before you need to cook the turkey.
Remove the neck and giblets from the bird (don’t throw them out, as you can use them to make stock for the gravy).
Using poultry shears or a heavy sharp knife, cut along each side of the backbone through the ribs to completely cut out the spine. Remove the wings as well, as they can get in the way and usually dry out during the cooking process (these can be useful in turkey stock, too).
Flip the bird so the meat is facing up and press down on the breast. Slightly crack the breastplate and flatten the bird. The flatter the bird, the more even the cook will be.
Mix the garlic powder, seasoning salt, sugar, rosemary, thyme, chili powder, and paprika in a bowl. Season the turkey on all sides with a nice, even coat. Let sit, covered, in the fridge for a minimum of 8 hours and a maximum of 24 hours.
Optional step: If you do decide to add smoke, only use it for the first 30 minutes, and only a trickle. Set up a smoker box on the side of the flame on your BBQ. Fill it with a nice fruitwood, like cherry. Cherry can give the poultry a nice mahogany color. Any more can overwhelm the flavor. For a more traditional turkey skip the smoke altogether.
Your turkey should sit on an elevated rack above a pan (not in it), for airflow on the top and bottom. The pan will help block the direct heat from the burners and keep your grill clean.
Preheat the BBQ to 275 degrees F (135 degrees C). Place the turkey on the indirect heat side of the BBQ (the side with no burners turned on) for 1 hour. After this, turn up the heat to 350 degrees F (175 degrees C) until the center of the breast reads 160 degrees F (70 degrees C) internal temperature. The turkey thighs should read approximately 170 degrees F (75 degrees C) in the thickest part of the meat. Times will vary according to each bird and its weight. Total cook time should be around 2 hours for a medium bird (see note).
Remove the turkey from the heat and let it sit in an insulated box (like a microwave not turned on) for 30 minutes minimum to let the juices settle. If properly insulated, the turkey can hold hot for 3 hours. Do this and you will free up more time to cook all your other dishes.