What PF said. Also verify that it’s a 20 amp circuit before using a 20 amp GFCI. Another tidbit, how old is your house? That outlet may already be consoled by a GFCI oulet in the garage. On most homes built in the last 15 years or so all exterior outlets are on the load side of the garage GFCI.
If you have a Dremel kinda thing (rotary tool, I have the M12 rotary tool), you can grind a slot in the middle, going perpendicular to the existing remains of the slot and try again with your screw driver.