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.