Highpass sharpening script (this is my first script for Gimp).
This technique is described by Rolf Steinort in episode 164 of the Meet the Gimp! video podcast.
You can run it from: Filters -> SkYNeT -> Enhance -> High Pass Sharpening
Because script makes a new layer from visible, make sure you have visible only layer you wish to sharpen. Other layers must be non visible.