This filter is found in Image>Filters/Enhance/Unsharp Mask

Out-of-focus photographs and most digitized images often need a sharpness correction. This is due to the digitizing process that must chop a color continuum up in points with slightly different colors: elements thinner than sampling frequency will be averaged into an uniform color. So sharp borders are rendered a little blurred. The same phenomenon appears when printing color dots on paper.

The Unsharp Mask filter (what an odd name!) sharpens edges of the elements without increasing noise or blemish. It is the king of the sharpen filters.

Some scanners apply a sharpen filter while scanning. It's worth disabling it so that you keep control on your image.

To prevent color distortion while sharpening, Decompose your image to HSV and work only on Value. Then Compose the image to HSV. Go to Image/Mode and click on Decompose. Make sure the "Decompose to Layers" box is checked. Choose HSV and click OK. You will get a new grey-level image with three layers, one for Hue, one for Saturation, and one for Value. (Close the original image so you won't get confused). Select the Value layer and apply your sharpening to it. When you are done, with that same layer selected, reverse the process. Go to Image/Mode and click on Compose. Again choose HSV and click OK. You will get back your original image except that it will have been sharpened in the Value component