True, but some many of the tutorials are a good place to start, even if you're unfamiliar with GIMP. I know many who have said as much regarding RobA's Creating coastlines in GIMP.
I am currently writing a series of map tutorials guides using techniques common to many graphics applications. While there is a chapter at the beginning that covers the basic steps used in all the tutorials explained for using GIMP, Photoshop, Inkscape and Xara Photo & Graphic Designer 9, the tutorials themselves only mention which basic step is being used and how to apply it to a particular step, and not really referring to specific applications. That series is called 25 Quick & Dirty Map Tutorials Guides, which there are 4 books by me, and one by Schwarzkreuz. My first four books are: 25 Quick & Dirty Map Tutorials Guide, 25 Advanced Quick & Dirty Map Tutorials Guide, Hand-Drawn Map Tutorials Guide, and the Using 3D Elements in Map Tutorials Guide. The fifth book by Schwarzkreuz is specifically for GIMP and Photoshop, though I don't know what is covered. Those books will be available as printed manuals and PDF files and not scheduled for release until August 2014 - though some material may be released before that.
Edit: here's a link to that successful (not running anymore) Kickstarter that funded this project: 25 Quick & Dirty Map Tutorials KS
Here's a link to my G+ community that supports the project: 25 Quick & Dirty Map Tutoirals Guide community