Wedge a wad of masking tape inside the open end of the extension pole. Use a finger to push it down about 3 inches and pack it tightly.
Place the back of the gaff hook inside the cavity and drill a hole for a screw across the pole, 2 inches from the end and a bit off center.
Fit the screw through the hole and secure it with the nut. It'll act as an anchor for the gaff hook and add strength once it's bonded in place.
Mix enough epoxy to completely fill the cavity, then pull the gaff hook up to the screw and hold it straight while the epoxy hardens.
Cut a 10-inch length of plastic tubing with an inside diameter that fits snugly over the hook and make a slit through one side, about 1 inch from the end.
Insert the hook into the end of the tubing and out the slit, then slide the tubing down the hook shank to rest against the end of the gaff handle.
Pull the end of the tubing up to the gaff-hook point, measure the length needed to cover the first 11⁄2 inches, and then cut off the excess.