Actually one suggestion I have is to browse the internet for GIMP tutorials. And I mean any tutorial - not specifically map ones. Tutorials on how to make flaming text, or some random pattern, or a reflective sphere or whatever.

Even if you're not interested in the end product it doesn't really matter. The whole idea is to become familiar with the various features of GIMP and you'll usually come away knowing a few extra functions with every tutorial you do. This is pretty much how I learned to use photoshop.