Textures are very easy to use in GIMP. Once you find a texture (or textures) that you like you can make patterns out of them that you can use to fill areas. Here is a quick step by step:

1. Find a texture that you like. (just a note on this step, GIMP can't "scale" patterns like Photoshop so you have to change the size of the original image to fit what you need. It helps to start out with a large image that you can make smaller)
2. Once you find the image and scale it to your desired size save it as a .pat file.
3. Restart GIMP and your new pattern will show up in your pattern dialog box.
4. Now you can use the bucket tool to apply your new pattern.

I hope this helps. Your on your way to a great looking map. I'm looking forward to watching this progress