You will see shades of red as the fill. This is normal when the mask is being viewed. View Mask to uncheck that setting. You should now see the a fading transparency layer in the selection frame. I used a dark reddish color.
Flood fill the backfill layer.
I used these settings, but you can use whatever looks good with your image. Add a new layer. This time, I used want the selection to be 50 pixels from the edges. Use the appropriate numbers for the dimensions of your image. Just take the width and height and minus 50 for the right and bottom settings.