Script-Fu -> Edges - Fade Outline
This is an adjustable script,
that fades the edges of an image.
You can change the width of the fade in pixels....
start and end percentages
And whether it fades to the background or transparent.
; This script should be placed ; in the user or system-wide script folder. ; ; Windows Vista/7/8/10) ; C:\Program Files\GIMP 2\share\gimp\2.0\scripts ; or ; C:\Users\YOUR-NAME\.gimp-2.8\scripts ; ; Windows XP ; C:\Program Files\GIMP 2\share\gimp\2.0\scripts ; or ; C:\Documents and Settings\yourname\.gimp-2.8\scripts ; ; Linux ; /home/yourname/.gimp-2.8/scripts