Place diced potatoes into the inner pot of the pressure cooker. Mix salt and garlic into water and pour over potatoes.
Close the lid and make sure the vent is set to closed. Set pressure to high and set for 10 minutes.
After timer is done, quick release pressure. If the steam is splattering, set a towel over the vent loosely.
Pour potatoes out into a colander and drain. While potatoes are draining, add butter, milk, sour cream, pepper, and garlic salt to the pot of the pressure cooker. Return potatoes to the pot and mash with a potato masher until smooth and creamy. Taste and add more salt and pepper as desired.