This script places a subtle reduction in brightness at the periphery of an image using a radial gradient. The method is non-destructive and customizable in that it creates a new layer that can be deleted or modified to suit personal preference. The intent is to save a few button clicks on a frequently repeated processing task.
(1) duplicate the active layer
(2) set the new layer's mode to "multiply"
(3) add a layer mask to the new layer
(4) draw a radial gradient on the mask from the center (using an offset value of 40)
(5) set the new layer's opacity to 60
Adjust the layer opacity in the layers dialog to suit your personal preference (or change the default opacity value in the script).
After copying this file to ~/.gimp-2.x/scripts/, restart GIMP and look for Filters->Light and Shadow->Subtle Vignette.
GIMP Guru: "Simple Vignetting Using the GIMP"
Luminous Landscape: "Digitally Simulating a Center ND Filter"