I'd start by un-mounting the motor from the gear cage.
Then take a look at how it's setup.. appears to be held in place via C-clips and the gears are secured via Roll pins.
Use a punch to back out the roll pins and once the C-Clips are removed they should just slide out.
Alternatively, spray some degreaser all over the gearbox, and then use a rag and toothbrush to clear everything out