Take the loose end of the remaining 8-inch black wire and connect it to the black wire on the dimmer switch, using a wire connector.
Attach the dimmer's yellow wire to the black wire coming from the newly installed light fixture.
Connect the bare copper ground wire coming from the GFCI to the dimmer's green grounding wire, using a wire connector.
Connect the final white wire to the dimmer's white wire, using a wire connector.
Carefully fold the excess wires into the box, then secure the GFCI and dimmer switch with the screws provided.