Is there a way to select an area in GIMP, then fill every part of the selected that is a certain distance or more from the edges of the selection other than doing it manually?
So a black circle like this:
Would/could be turn(ed) into a black circle with a dark gray circle on the inside:
Which could be performed multiple times, turning it into this:
If the object is on a transparent background this can also be done (a bit more accurately) using the ofn-outline-layer script.
If it's arbitrary shapes, continue with the process above, selecting the inner shape. You may have to Select>Grow your selection by one pixel each time to avoid a tiny gap. The advantage of using a path to draw the border is that you can keep sharp angles (see the "Join" option in Stroke path), so your inner shape still looks like the outer one (otherwise the inner shapes will turn into an nondescript blob).
If it is just plain rings, the good way is to stack decreasing rings. Position the center using two guides, and starting from this, create an Ellipse selection by forcing the aspect ratio to 1:1 and using the Expand from center option. In other words think of you image as this:
