Paste as New
Below I describe a simple way to to take a selection and make everything else transparent. Sometimes this is a lot less effort than fiddling around with a difficult background and other disparate elements in an image. It also hints at the next entry in the "transparency" section here at gimphelp.org, where we find out just how simple a concept, yet how powerful, layers really are . . .
- finish making your selection -- here I activated the "lasso" tool, pressed Shift (to add to selection) and proceeded to round up the windows in the factory as to include them with the rest of the building.
- Choose Edit>Copy for your selection
- Choose Edit>Paste as New
- New image will have your selection on a single, transparent layer.