Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Heat olive oil. Sprinkle garlic powder, salt, and pepper all over the pork loin and sear in the hot oil on all sides, about 2 minutes per side. Remove pork from the pot. Pour in chicken broth and scrape up all the browned bits so none are stuck on the bottom. Turn off Saute function.
Pour in apple cider and add pork loin back to the pot. Mix ground thyme into the applesauce and spread over the top and sides of the pork loin. Place onion slices around the pork loin. Close and lock the lid. Select High pressure according to manufacturer's instructions; set timer for 25 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure using the natural-release method according to manufacturer's instructions, about 15 minutes. Unlock and remove the lid.
Remove pork and onion slices from the pot. Mix cornstarch with cold water in a bowl.
Turn on Saute function and stir cornstarch mixture into the pan juices. Bring to a boil and cook until gravy has slightly thickened, 3 to 5 minutes. Serve gravy with the pork loin.