I haven't actually used Gimp (I don't like free-floating windows, it's entirely subjective), but assuming that it has similar functions to photoshop in terms of possible rendering tools and Alphas Channels, it should be entirely translatable. Some parts of the process are translatable into ArtRage as well, though not all.
Thanks for this tutorial series. I'm fairly comfortable with the techniques I've used in the past for map making, but it's always good to see the techniques others have developed. These tutorials will definitely give me some alternatives to play around with.