Combine GF breadcrumbs, egg, milk, onion powder, salt, and pepper in a large mixing bowl. Let milk soak into crumbs for a few minutes.
Gently stir in ground beef/turkey until well blended. Form into balls about 1 to 1 1/2 inches in diameter.
Brown meatballs in 1 Tbsp olive oil in a large skillet; remove with a slotted spoon to a 1 1/2 quart baking dish. Drain off all but 1 Tbsp of drippings.
Stir GF flour into drippings. Cook, stirring constantly, until bubbly.
Stir in beef broth and cream (can also add sour cream). Continue cooking, stirring constantly, until sauce thickens and boils for a minute. Pour over meatballs.
Bake meatballs at 325° for 15 minutes. Serve over GF noodles or rice.