Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add onion and celery; saute until tender.
Combine sauteed vegetables, ground round, and next 5 ingredients in a large bowl; stir well. Shape mixture into a 9- x 5-inch loaf; place on a rack in a roasting pan coated with cooking spray. Bake at 350° for 1 hour and 15 minutes.
Combine beef broth, flour, salt, and ginger in a small saucepan; stir until smooth. Gradually add milk, stirring constantly. Cook over medium heat, stirring constantly, until thickened. Remove from heat, and add sour cream, stirring just until blended. Spoon evenly over meat loaf.