This filter maps active layer or selection against Toolbox gradient: it applies Toolbox gradient colors to the image. First, you have to select a gradient in the Gradient Editor , either by clicking on gradient area in bottom right corner of Toolbox or by Image>Dialogs/Gradients. This filter uses image color intensities (0 - 255) and connects darkest pixels to Gradient left end color and lightest pixels to Gradient right end color. Intermediate values are set to corresponding intermediate color.
This filter is found in