Sauté: Start by setting your Instant Pot to sauté mode and melting the butter. Add the onion, garlic, and celery, and cook until softened, about 2-3 minutes. Stir in the garlic powder, dried thyme, salt, and pepper.
Add chicken: Pour the chicken broth into the Instant Pot, scraping any browned bits off the bottom. Add the chicken breasts, ensuring they are covered with broth.
Pressure cook: Seal the lid and pressure cook on HIGH pressure for 6 minutes (adjust to 8-10 minutes for larger chicken breasts). Once done, allow the pressure to release naturally for 5 minutes, then manually release any remaining pressure.
Shred chicken: Open the lid, remove the chicken, shred it, then return it to the soup.
Finish soup: Switch the Instant Pot to "sauté" mode. Whisk the cornstarch into the milk, then stir it into the soup. Add the carrots and gnocchi, and simmer until the gnocchi are done (this should only take a couple of minutes). Finally, add the spinach and allow it to wilt for one minute before serving.