With the gearbox fitted to the motor, you should be able to turn the motor with the white gear by hand. The gearbox is a nylon reduction drive that is made up with planetary gears inside.
If the motor wont turn by hand you will need to strip it down to see why, there are a number of reasons, one is the magnet stator peels away from the motor body and jams on the commutator, but untill you take a look it could be anything.