Step 1
Make a new raster layer.

Step 2
Take the selction tool. and Select whatever you want on your picture. I’m using the Rectangle as my selection type. Don’t worry about the other settings. Now after you make your selection, take your flood fill tool and choose the color white (or whatever color you want) and fill in the selection.

Step 3
Now, click Layers>>Properties. And change around the Opacity to as much as you want.

My Result