Roasts can go straight onto the grill. There are some reasons, however, that you would want to put one on a rack over a pan.
I would recommend you watch for bonless pork loins to go on sale in the $2.00 per pound range and pick one up.
Trim the top layer of fat and season it generously with:
2 T. Salt
1 T. Garlic powder
1 T. Onion powder
1 t. ground thyme
1 t. ground bay
1 t. rubbed sage
1 t. ground mustard
1 t. fresh ground black pepper
Mix these really well and put them in a bottle with a shaker top - preferably one that will have a lid over the holes.
Fire up the grill for roasting 250-350 d.F. Rub the mixture on all surfaces of the pork loin. Place a rack over a shallow pan and put about 1/4" water in the pan. This is to keep the early juices from burning. By the time it boils away, it will be replaced by drippings.
Put the loin section on the rack, close the lid and go play for about 45 minutes. Come back and check the internal temp. with your trusty thermometer. Take it off when the internal temp. reaches 145 degrees. Let it set for 10 minutes while you take the drip pan to the stove, add 1/4 cup water, bring to a boil and scrape all the juices off the bottom of the pan. Add a little of the original seasoning - to taste. Pour up in a bowl and serve on the side.
Slice the loin thinly across the grain and get ready for a palate pleaser. If there is any left, cover it with the gravy and refrigerate. It makes great sandwiches.