You can find this filter through
→ → .This plug-in calculates the most often used color in a specified border of the the active layer or selection, allowing a specified range to not be exact. The calculated color becomes the foreground color in the Toolbox. This filter is interesting when you have to find a Web page color background that differs as little as possible from your image border. The action of this filter is not registered in Undo History and can't be deleted with Ctrl+Z.
You can set there the border Thickness in pixels.
You can set there the Bucket Size, i.e a number of color levels added to, or subtracted from, every RGB channel (1 - 256), making the calculated color tending to gray.
Examples illustrating the “Border Average” border: