Combine egg, milk, salt, pepper, and garlic salt. Dip grouse into mixture. Dredge grouse in flour. Add butter to skillet and heat to frying temperature. Brown pieces of grouse well on all sides. Remove from skillet and place in a single layer into a baking pan. Add skillet drippings and water to mushroom soup and mix well. Spoon over grouse. Cover with aluminum foil and bake at 350 degrees for 1 hour or until tender.