I don’t use GIMP (however I am well versed in Photoshop and much of that translates) but some general art tips I’d employ is to use Google to look up a few good photographic references and get ideas for exactly how you want things to look, colour, lighting, environment and cloud-wise. Also, make sure if you aren’t already to use layers – they make life soooo much easier with digital art!
A great place to look for digital art help is deviantART – just use dA’s search engine and search ‘sunset tutorial’ or ‘sky tutorial’ and I’m sure you’ll find lots of great advice. 🙂 (Nicely visually depicted too!)
I learned photoshop mainly just with those tutorials – some of them are really awesome!
Check out my finished artwork at http://falcolf.deviantart.com/ and my sketch/studio blog at http://rosannapbrost.tumblr.com/
Excellent!