You can find this filter through
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
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 which pops up as the Foreground color of the Toolbox.
Invert FG/BG colors so that BG color is the same as Web background
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.