Preheat oven to 400 degrees F (200 degrees C). Grease a 9x13-inch baking pan with cooking spray.
Mix pumpkin puree, eggs, milk, vegetable oil, and honey together in a small bowl.
Combine flour, cornmeal, baking powder, poultry seasoning, salt, and garlic powder in a large bowl. Pour in pumpkin mixture; stir until well blended. Fold in onion and celery. Spread batter in the greased baking pan.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 40 to 45 minutes. Cool briefly before slicing, about 10 minutes.