This filter is found in
Filters->Colors->Border Average.
This tool calculates the average color in a border around active layer
or selection. It is interesting when you have to find a Web page color
background that differs as less as possible from your image border.
Options
Border Size
You can set there the border Thickness in
pixels.
Number of Colors
You can set there the Bucket Size, i.e the
number of colors to be used when calculating the average color
(2 - 128).