When you use a non-stick roasting pan, most of those flavor-packed brown bits stay on the meat rather than sticking to the pan. Good for the taste of the roast, not so good for the taste of the gravy.
What you can do is toss some cut up veggies into the pan and roast them along with the meat. Chunks or thick slices of onion, carrot, celery and garlic will caramelize and pick up some of the juices from the meat. They can serve as the base for a tasty gravy.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan