Hehe, that's a fun idea! I know next to nothing about GIMP and its capabilities, but a couple of general things. First I think you've got it pretty much right with the shape and character. It just needs perhaps a couple of tweaks:
His left ear (as in the left ear of the character, I'll use that view point) seems a little high, but if that's intentional for the trolly look, then don't mind me
The nose and mouth area could use more definition, especially more darks. In fact, a bit of oomph in the values wouldn't go amiss in general. Perhaps some slightly yellow-toned brighter highlights and a cooler pass over the darks.
Also consider your light source - it looks to me like it's coming from his top right, which means the shadow should be that bit more pronounced on the bottom left of the face and collar.
Linear Mode
