Pour chicken stock into the bottom of inner pot. Add chicken, then sprinkle 1 teaspoon of the garlic salt, plus the poultry seasoning, over top.
Add celery, carrots and onion on top of chicken. Place lid, lock and set pressure valve to SEAL.
Cook on HIGH pressure for 25 minutes. When timer has elapsed, allow to naturally release for 10 minutes before quick releasing remaining pressure.
Use tongs to remove chicken from pot. Shred and set aside.
Cut biscuits into quarters and sprinkle with remaining teaspoon of garlic salt. Make a cornstarch slurry by mixing the milk INTO the cornstarch and stirring until smooth.
Press CANCEL on pot, then SAUTE. Pour in the slurry mixture and cut biscuits, then stir. Loosely place the lid on pot (allowing steam to vent) and bring to a simmer for approximately 10 minutes, until thickened and dumplings heave doubled in size.
Stir in reserved shredded chicken, then ladle into bowls and serve.