Lift the gagging (hacking) cat gently and support its chest with one hand, and (again very gently) grasp the hind legs. Spead you legs and while supporting the cat mainly by the chest hold, using the hind leg hold primarily for steering, swing the cat back and forth between your legs like the clapper of a bell. The obstruction should just pop right out. NB: The cat will probably be very dizzy afterwards, so help it to its pillow.