I always leave the gearbox on, and like Baird444, I remove the primary chaincase before removing it. I also take off the head and barrel, so that I have maneuvering room, and to make it all lighter to handle.
On the bottom of the gearbox are 2 slots/hooks which drop on to the cross-bolt that goes across the frame under the gearbox. You have to remove the top triangular gearbox plates, and then you have to lift the gearbox up enough to get the hooks off the crossbolt. Then you can rock the engine out by lifting it up and out the primary side.