Preheat oven to 375 degrees F (190 degrees C). Spray a 9×13-inch baking pan with cooking spray.
Heat a large skillet over medium heat. Spray lightly with cooking spray. Add chicken strips; cook until no longer pink in the center and the juices run clear, about 4 minutes per side. Cut into cubes.
Transfer chicken to a large bowl. Add chicken gravy, cream of chicken soup, evaporated milk, stuffing mix, onion, garlic, celery seed, and black pepper; mix until combined.
Spread frozen potato nuggets in the bottom of the prepared pan. Pour chicken mixture evenly on top. Sprinkle mozzarella cheese and Cheddar cheese on top.
Bake casserole in the preheated oven until golden and bubbly, about 45 minutes.
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