Load the crockpot: Add the chicken, carrots, celery, onion, broth, cream of chicken soup, garlic powder, poultry seasoning, salt, and pepper. Stir to combine.
Cook: Cover and cook on low for 6 hours or high for 3 hours, until the chicken is tender.
Shred the chicken: Remove the chicken, shred it with two forks, and return it to the pot.
Mix the dumpling dough: In a bowl, whisk together the flour, baking powder, and salt. Stir in the milk and melted butter just until combined.
Add the dumplings: Drop the dough by spoonfuls onto the top of the simmering broth. Cover and cook on high for 30-45 minutes, without lifting the lid, until the dumplings are cooked through.
Notes
Critical technique: once the dumplings go in, keep the lid ON for the full 30-45 minutes. Lifting it lets steam escape and dumplings turn out gummy.