Gybing with an aft mainsheet is slightly different. As with tacking, you change hands on the mainsheet and tiller extension before the gybe and cross the boat facing aft.
Do the pre-gybe checks, including checking the area is clear, and warn the crew you are about to gybe by calling “Stand by to gybe”. Change hands on the mainsheet and extension. Call “Gybe-oh” and push the tiller to windward to start the turn.