Use this option when you need the person in the photo looking in the
    other direction, or you need the top of the image to be the bottom. Use 
     →  →  , or use the 
    
       button on the toolbox. After selecting the flip tool from the
    toolbox, click inside the canvas. Controls in the Tool Options dockable let
    you switch between Horizontal and Vertical modes.
     button on the toolbox. After selecting the flip tool from the
    toolbox, click inside the canvas. Controls in the Tool Options dockable let
    you switch between Horizontal and Vertical modes.
After selecting the flip tool from the toolbox, click inside the canvas. The tool flips the image horizontally. Use the options dialog to switch between horizontal and vertical. If it is not already displayed in the dock under the toolbox, double click the toolbox button. You can also use the Ctrl key to switch between horizontal and vertical.
In the images below, all possible flips are demonstrated:
Abbildung 3.36. Beispiele fürs Spiegeln
 
              Originalbild
 
              Horizontal gespiegelt
 
              Vertikal gespiegelt
 
              Horizontal und vertikal gespiegelt