Well, there is the tutorial tag here, but my favorite places for digital painting tutorials are deviantArt, conceptart.org (though often too advanced for my level of skill), and the imaginefx.com site offers many of its magazine's tutorials for free on their website. Mostly these aren't for GIMP but you can adapt the techttp://conceptart.org/forums/forumdisplay.php?f=42hniques.

ETA: To be more specific, here is a link to conceptart.org's tutorial forum:
http://conceptart.org/forums/forumdisplay.php?f=42
here to imaginefx's tutorial's (called "workshops"):
http://www.imaginefx.com/-2287754330326480692/Workshops.html
and for dA just search for the topic in the tutorial section.