I'm not familiar with GIMP, but I can suggest you a manual technique...
Well, first select a portion of the image that you can use to "patch" the problematic area...copy it. Then create an empty layer, and paste that portion on the new layer and place it on the problem area.
Now, grab the eraser tool (or add a mask on the new layer, if you can. That's better) and delete the surrounding area with a soft brush. If you use a too soft brush, you will loose texture around the area, this is an unwanted situation. After cleaning the surrounding, grab the dodge and burn tool. Use them to equalize the light of the new layer. You can also use "curves" to equalize the light too.
You can use GIMP's clone tool and then use this method to clean the mess, too.
Believe me, Photoshop's clone tool does not create wonders like they show in the tutorials. It's good, but generally not enough for stock quality (perfection).
Hope it helps :)
- Nikkor 50mm f1.4
- Nikkor 50mm f1.8
- Tamron 90mm f2....