As Ascension said, you might be better off with a vector graphics program like Inkscape rather than a raster one like The GIMP. At least initially.

I've created a template for a world map here: http://www.cartographersguild.com/sh...447#post154447

It's an SVG file for Inkscape, but it can be loaded into other programs, including the GIMP though you will loose the benefits of vector graphics in doing so.