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. Select
     →  →  from the main menu, 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.
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:
Рисунок 3.26. Example Image to Flip
 
              Source image
 
              Horizontal flipped image
 
              Vertical flipped image
 
              Horizontal and vertical flipped image