And if you haven't got a grad filter you can create one in GIMP.
Duplicate your background layer and use curves to correct the exposure for the bottom of the image. Don't worry about how washed out the sky looks.
Add a layer mask to the lightened layer. Put a black to white gradient on the layer mask starting just above the horizon. This will leave your original sky but gradually bring out the foreground. Try it starting at different heights above the horizon until you get what you are looking for.
Hope this helps.