Gimp actually works very similar to Photoshop, except that Photoshop has a different user interface.
One of the most important things you need to know, is how you work in layers so that you don't have to start over if you make a mistake or don't like your result. In fact, my best advice to you, is to make a new layer for every major change you make to the image and LABEL your layers so that you can find them easily. There are a few tutorials about the net that give you the basics of Gimp use; I found this one via google Gimp Basics List There is also a breakdown of how to do layers there as well. I would suggest that you use the links at your own discretion, but the Youtube ones are almost 100% safe and those already include all you would need to retexture.
(Ironically the main GIMP website is down, though)
You can also hopefully find tutorials through a google search as well; though once again, you have to enter websites at your own discretion, but enough about that...
Personally, what I do to retexture is to either run a couple of filters to change the color or to paint a layer over the existing one and change its blending mode, but I'm sure you will develop your own system.
I also make sure to have references in order to keep my work looking consistent with canon and the look I'm trying to convey.
If you need any assistance or tips, feel free to PM me in the meantime.
Edit__
I personally am also interested in what ideas you have for the robes. Could you maybe post a few of the reference images and ideas you have?