Ground wire gauge for 15 amp duplex outlet on 20 amp circuit

I am replacing an ungrounded duplex outlet with a grounded one by grounding to the box. It is a 15 amp outlet on a 20 amp breaker. There are multiple outlets on the circuit. Should I use 12 or 14 gauge for the ground wire connecting the outlet to the box?

Edit for clarity: The boxes in this case have been tested and are conductive to ground. I am doing 10 of these so gfci would be a last resort due to cost.