I know this question may have been already answered, but I didn't find a satisfying answer...

I drawed a map of a large continent, and I want to make it look like this. It's a map of the Wheel of Time serie:


I found on the forum a lot of ways to make fantasy-like maps of fantasy worlds, but not an atlas way to do it. My other issue is that I use Gimp ( I saw a lot of guys using phtoshop, which I never did). Is there any way it can be possible?

Thank you for your support