I'm replacing our kitchen countertop 20-amp small appliance outlet with a GFCI outlet. This outlet is the only item this circuit.

I removed the old outlet to find only two wires, the red hot wire and the white return wire. The box is metal, connected by flexible armored cable and metal conduit to the service panel (so I assume that connection is the only ground).

But ... there's no grounding terminal in the outlet box for me to wire the GFCI's green grounding terminal to.

The GFCI installation instructions do not state whether it is necessary for the receptacle's grounding terminal to be attached to 'something' for it to function properly.

Should I 'create' a grouding terminal in the box myself by driving a screw into one of the small holes in the back of the box?

Or will screwing the GFCI to the box create a sufficient ground connection?

Thank you.