I don't use GIMP, so I can't really comment on the nuts-and-bolts, but the thing that stands out to me here that could use improvement are the rivers. They have too many straight areas and almost right-angles, especially that river flowing from Embrium to Gal Hesperion. The other thing that bugs me river-wise is Capsan Island. All those rivers crammed in like that on an island just looks 'off' to me.

I do, however, like the landmass shapes, the river placement (except for the points above), and the general color/texture of everything. I also dig your place-names. Have some rep!