This filter is found in Filters->Edge detect->Sobel.
Sobel's filter detects horizontal and vertical edges separately on a grey-level image. Color images are turned into grey-levels.
Sobel Horizontally: Renders near horizontal edges.
Sobel Vertically: Renders near vertical edges.
Keep sign of result: This option works when only one direction is selected: it gives a flat relief with bumps and hollows to the image.