Prepare noodles according to package directions, omitting salt and oil.
While noodles cook, place beef, onions and mushrooms in a large nonstick skillet, cooking until meat is browned. Stirring until beef crumbles. Drain meat.
Return meat to skillet. Add gravy, sour cream, garlic salt and pepper. Stir well.
Cook over medium heat 5 minutes or until thoroughly heated.
Serve over drained noodles, top with fresh ground pepper.