It's better than some of Photoshop's selection tools GIMP's Foreground Select tool doesn't get enough love, but it's one of the software's best secret weapons, unlike Photoshop's Quick Select and ...

Click the "Bucket Fill" tool and then click inside the selection you drew previously around skin in the target image. GIMP fills the selection with the foreground color.

If you can’t select the foreground objects or the background, you can’t edit much of anything. GIMP has seven Selection Tools, while Photoshop 2020 has ten.

You can then right-click the "From" text box in the Color to Alpha window and select "Foreground Color." When you click "OK," GIMP makes that color transparent.