Landmasses shapes are ok and interesting. Graphically, you should, I think:
•*add some kind of texture and/or colour variation to the land and the ocean
• use a more subtle stroke for coastlines
• vary the appearance of mountains (and possibly have a little less of them) and work on blending them better with plains
•*do without the "blurring" that afflicts some parts of the map but no other (why?)
•*remove the white "mist" under the text, you could also do with a much smaller size, it would better convey the vastness of the geography (there is no scale, but since there are three oceans in the map, I guess it is at least an Africa sized continent).

I never used GIMP so I don't have any more technical advice to give.