To clone a piece of an image to another part of the image...

• Open the image in GIMP

• Click on the clone tool (looks like a rubber stamp)

• Hold down Ctrl then click the brush to select the area you want to clone (resize/change brush as required)

• Use the mouse to "stamp" the cloned area to other areas of the image (changing opacity can improve this, experiment)

• Click File then Save... / Save As... / Overwite / Export As... as necessary