Nice work so far.
I'm not an expert with gimp (yet) but I think if you put in some simple borders like ------ or <><><><> (or anything else really) and put the names of your countries in once you figure them out, the color fill won't be needed as much. You could try to establish some fancy border, but that might not go with your map.
In terms of fonts -->
"Corbel" is a nice smooth font you might be interested in for this style. However, if you want a fancy non-disruptive look, "Blackadder ITC" is always a nice choice.
Much luck to you!