This filter is found in the image menu via Filters → Colors → Semi-Flatten It is available if your image holds an Alpha channel (Image>Layers/Transparency/Add an alpha channel). Otherwise, it is greyed out.

The Semi-flatten filter helps those in need of a solution to anti-aliasing indexed images with transparency. The GIF indexed format supports complete transparency (0 or 255 alpha value), but not semi-transparency (1 - 254): semi-transparent pixels will be transformed to no transparency or complete transparency, ruining anti-aliasing you applied to the logo you want to put onto your Web page.

Before applying the filter, it's essential that you should know the background color of your Web page. Use the color-picker to determine the exact color. Click and drag this color from the Web page to the Toolbox ForeGround color. Invert FG/BG colors so that BG color is the same as Web background color.

Semi-flatten process will combine FG color to layer (logo) color, proportionally to corresponding alpha values, and will rebuild correct anti-aliasing. Completely transparent pixels will not take the color. Very transparent pixels will take a few color and weakly transparent will take much color.

This filter has no options.