Good morning.
This is relatively...no it's actually very easy thing to do.
What you need, is either a big 2-colored world map, or a vector map.
There's a world map in wikipedia. SVG is the native vector format of Inkscape, and you can open/export the SVG paths into Gimp.
https://commons.wikimedia.org/wiki/File ... orders.svgDownload the svg. Make sure to download as .svg, not .png!
In Firefox it can be done by clicking the image and when it opens in browser, right-click the image and use "save page as".
Open in Gimp, in the export window tick both boxes, import paths, and merge paths.

Switch to paths, right-click the path, "path to selection". Create new layer and fill.

If you wouldn't have a ready vector, a big 2-color .jpg or .png would require just a small additional step.
Menu: Select/By color, click the land color. Select/To path...and so forth.
With .png you could also use Layer Effects: Color overlay to change the color, or pattern overlay to overlay the land with pattern, but this requires additional plug-in.
A ready vector will always be more handy and accurate.
Let me know how it goes, or if you need more help.