Offset a stroke

[Ofnuts]

It is actually easier to create the shape at full size, and then stroke an outline over it.

For example like this:

   

   

In practice,

• create a selection with the shape of the green disk
  
• add a layer and bucket-fill it with green
  
• Select > To path
  
• add a layer and Edit > Stroke path with red
  

In you case, that would also use less ink, and reduce the area with two layers of ink.

If you want to stick with your technique, you can instead use Select > Shrink to shrink the shape by N pixels. This is almost an offset, but this tends to round off corners. To keep sharp corners, you can get a path from a the selection, stroke it as above (using miter joins in the line style), and get a selection from the stroked line.