For more detailed info on how the Clone and Heal tool work in GIMP, check out this tutorial.
Before using either the Clone or Heal tool, you need to let GIMP know what part of the image will be used for retouching. Grab your Clone tool (C) and try to apply it to a layer. Nothing happens.
To target an area to copy from, hold down your Alt or Option key and click the part of the photo where you’d like to copy from. When you do, the shape of your brush will be left behind.
In this case, I’m using a round soft-edged brush, and I have a (dashed) circle left where I clicked. As you apply your brush to the image layer, the target circle will follow your brush strokes.
Once you release your mouse button, the target circle will revert back to the original target area. To remove the dress, we’ll target the stairs to be used to clone over our subject.