Spray 1 1/2-quart slow cooker with cooking spray. Place chicken in cooker. Top with onion, poultry seasoning, thyme, pepper, gravy and celery.
Cover; cook on Low heat setting 4 to 5 hours.
Gently stir frozen vegetables into chicken mixture. Increase heat setting to High. Cover; cook 30 minutes.
Meanwhile, heat oven to 375°F. Bake biscuits as directed on bag.
Spoon chicken mixture onto plate; top with split biscuits.