New guy here! Got brought in by all the wonderful tutorials lying around mostly.
So I was following through the Eriond tutorial, making tweaks here and there, and I ended up with this after messing about with details (mostly in Wilbur) for about a week to find a process I liked.
I followed the tutorial up to the part where the mountains are made and then I diverged a bit by having an overlay layer to do the elevation gradient with and making depressions for the lakes.
After I exported to Wilbur I messed around with the terrain to get a few lakes to fill just the areas I wanted just right then used the Fill > Compute Basin Deltas to get a height map for the lakes. (I also did a span of -100 to 3000 to keep the ocean below sea level [which is 0] so it wouldn't get rivers etched into it).
Then I undid that exported a height map for the general terrain before doing the next step.
First I filled the basins in with land using the Fill Basin filter and generated the rivers I wanted using some ridiculous settings with the Incise Flow filter (something like: Amount: 1000+, Flow Exponent: 5+, Effect Blur: 100+, and then Post Blur: 0.1 to make rivers downstream bigger than flow into it).
This utterly destroys everything else but etches some rivers nice and deep into the map, though before I exported I used the brush on the Maximum mode at a height of -70 to raise the rivers up from -1000 or lower so when I exported the greyscale height map it would turn out right.
I mostly followed the tutorial from there on, adding the rivers and the lakes into the River/Land mask, and used the gradients to color the land.
After I did that I went back and colored the water and oceans using the techniques from the "Using GIMP to Create an Artistic Regional RPG Map" tutorial with some airbrushing here and there.
I plan on adding in forests tomorrow using the "Photo-Realistic Forests in GIMP" tutorial.
If anyone wants the .xcf file I can upload it to Google Drive or Mediafire or something.