This script-fu (created mainly from the script "add-bevel.scm" which is installed by default) adds to an existing layer, a layer composed of squares or rectangles (size of your choice) in shades of gray but with bevels. The goal is to facilitate the creation of pavements with a little "volume". arfff. Difficult to explain -_-'
You can find "00 - Volume Express..for simple tiles" in Filters>2D Isometric-Fu>
A picture is better than a thousand explanations:
With a little work and once turned into isometry (with another script):
EDIT: New version of the script => v1.1