When it comes to sharpening an image, a popular alternative to the unsharp mask is the high pass filter. To apply this method, you create the high pass version of your image in a new layer and set its blend mode to overlay. Applying a layer mask allows you to restrict sharpening to certain regions of the image.
Unfortunately gimp does not provide the high pass filter out of the box. You either need to install gmic or the high pass plugin.
Gimp Magazine published its latest issue.