One of the secrets of great image manipulation is to separate images into components. Frequency separation is often mentioned, breaking the image into so-called high-pass and low-pass layers. The high-pass layer separates out the sudden changes in an image and fiddling with this layer can enhance sharpening or even out blemishes. The low-pass layer can be used for smoothing out color transitions. Portrait photographers use this extensively for smoothing skin and removing spots. There is no native Frequency separation tool or filter built into gimp.