So, a long time ago (in a galaxy far, far away), I ran into a tutorial by German artist Calvin Hollywood for using an inverted/surface blurred layer set to vivid light mode over the base image to greatly enhance details (local contrast).
The problem was, that at the time I couldn't get it to work in GIMP (due to a lack of Vivid Light blending mode, and no Surface Blur equivalent from photoshop).
Well, after talking with David the other day, he added a Vivid Light blend mode to G'MIC for me (there was already a Bilateral blur, which is pretty much exactly how PS does it's "Surface Blur").
So I was finally able to replicate the results from the tutorial, and have written a blog post tutorial detailing how to do it in GIMP:Getting Around in GIMP - Calvin Hollywood Freaky Details in GIMP
Then David went and actually added the effect as a filter into G'MIC directly... So the tutorial is instantly redundant!
Still, if you're interested in seeing a neat effect, check it out!