The very front of the gearbox has a series of threaded holes. I believe there's eight of them. One or two of those holes has a hidden set screw in it that prevents the gear box from unthreading. Pretty sure the balldriver size for the set screw(s) is 1.5mm. Put the 1.5mm ball driver down each hole to find the set screw(s) and loosen it (or them). You then need a pair of strap wrenches. Sears has some nice ones. Use one strap wrench on the motor and the other one on the gear box to unscrew the gearbox from the case. There's generally a little loctite on the threads so don't be surprised if it takes a fair amount of force.

Verne