This script sharpens an image with the built-in GIMP plugins erode and dilate.
These two plugins infiltrate the image with the lowest/highest pixel of the closer environment, but never higher or lower.
This means, they can be used to sharpen an image w/out halos.
Copy it into the scripts folder from GIMP, you will find it then under Filters → Enhance.
If you like it, hug someone.