Get a couple of very large flat screwdrivers or pry bars and wedge them between the gearbox and pump at 3:00 and 9:00. Try wiggling the bars back and forth to make the gear slide loose. If all else fails, you can heat the gear with a torch. Get all plastics (dipstick, sight glass, drain plug with o ring) off the housing. You will probably have to replace the seals in the gearbox and engine.