This filter is found in
Image>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).