I would think that the rubber O-ring is seized in place. This hold the ticker cylindrical part where the actual gear spins free within.
The round edge that you see when looking inside the drive gear hole actually has an edge to it.
Using two flat screwdrivers, you might be able to lever it out of there. One screwdriver at 6 o clock, and the other at 12 so to say, giving a equal lift and not skewing it in it's hole.
As the O-ring is likely the element that is stuck, you have to realize that it sits on a smooth surface to create an effective oil seal. While it may be tempting to gently poke a screwdriver between the O-ring and the housing to break it free, be carefull for scratches to the housing that may compromise later (re-)sealing of the speedogear. Either use something plastic, or accept that the gearbox housing may be ruined afterwards.