Unfortunately, there isn't a wide range of quality tutorials on GIMP, as opposed to Photoshop. I would probably look at the basic GIMP tutorials and then get a good idea of what you'd like to design (or what designs you'd like to learn to make).
Here's one I dabbled with some time ago. Just did some layer and light effects while tinkering with blending a couple of main pictures together:
You really need to have a grasp of Photoshop to understand Gimp. I haven't used Gimp in a long time, but from what I remember it's not a simple, intuitive program. For the money it's definitely a good purchase, it just takes a while to get going.