You can use a pin punch and peen the gear to the axle or drill and tap and install a set screw. This is a common problem on Dr. Who, bad design requiring too much torque on the press fit axle and gear.

I have taken several of these apart and soldered the gear back together that is giving you the problem. The best I can tell = this was made to "brake" - in the event you were  to maybe get your fingers stuck in the unit. This is only a guess. Repairing the gearbox requires drilling out the rivets which you will later have to replace with screws and nuts. The only way I have fixed these is when the gear on the main shaft is slipping. You will have to take the unit all apart and clean all the grease off the parts to get the solder to stick. You will also have to use a small butane torch also to heat the gear up. One last thing to note once this gear is soldered there is almost nothing stopping this gearbox from braking your fingers if you were to get them stuck in there some how. If you repair it this way be careful!!!