Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 146

In order to edit the scratched area more comfortably, use the Zoom Tool
(you can also call it up from the View > Zoom menu) to access a more detailed
view of the area.
To transfer certain picture information with similar but undamaged
content from one point to another, you have to be precise when you're
choosing the section in the picture and placing it on the damaged section.
Make sure you choose and repair important sections with edges, contours,
and distinctive elements first. Uniform surfaces are not so critical and don't
need precise picture information. They can be filled out easily.
Figure 2.68
Precise work is neces-
sary to repair distinctive
image elements.
Undoing a Step
The Undo (History) function was discussed earlier in section 1.5.6. If you
inadvertently clicked on the wrong area, just use the Ctrl+Z keyboard shortcut
(or choose Edit > Undo ) to undo a step. In the image window under Edit >
Preferences > Environment , you should have already defined the number of
steps you can undo ( Minimal number of undo levels ).
Alternatively, you can open the Undo History by clicking on the tab Undo
History in the Dialog Dock window or by choosing Windows > Dockable Dialogs
> Undo History in the GIMP image window.
In addition to removing blemishes with the Clone Tool, you can remove
unwanted elements from an image. Don't forget to save your image when
you're finished.