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 Tools → Transform Tools → Flip, 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: