I'm not a GIMPer so I'm not sure what it is capable of but the mountains need some height and the rivers have a light blue stroke outline...I'd change that to a medium green to accentuate the floodplain around the river and also make it wider and blurry. Of course at this scale I never put a floodplain in or a beach 'cuz it's very hard to get them to look right. All in all this is a very nice start and if my Photoshop skills can help I'll see what I can add but others here have mad GIMP skillz. I wrote a tutorial on doing continents that might be of help but it's for Photoshop and Bryguy replicated it in GIMP so check out the tut section (although the colors used there are darker you can always change that).