I tried my hand at creating a map in GIMP. I followed a six step tutorial from The Coffee Bean GM. Here is his playlist if you want to try your hand at map making. Here are my creations after finishing each video.
Land and water creation
Added texture
Some lighting on the map
Land features added
Land features coloured in
Final creation with added rivers, roads, locations, and other features.
This was a fun project. I hope you take the time and make something of your own. It will get you familiar with GIMP and will get your creative juices flowing.
Have a good one. Peace.