Click the icon at bottom right (circled in red) to create a new layer.
Select 'Ellipse Select' tool, to create an ellipse on the new layer that fits the picture.
Right click, choose 'Select Inverse', select the area outside of the ellipse(the edge).
Right click again, choose 'Feather' tool, input '100' pixels (smaller figures for lower pixels pictures).
Choose 'Paint Bucket' tool, make sure foreground color is black, then paint the selected area to black.
Deselect all area, then change new layer's mode to 'soft light', set opacity to preference percentage. Done!