Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 198

painting over the surface, you can reselect image data. This time, clone some
data from another point, such as, for instance, from above the billboard.
Alternatively, you can go back a step in the Undo History and correct the
cloned perspective.
Figure 3.24
The completed image