That looks great, I've never even heard of Gimp, might take a look.
Just something that i noticed might make it look a bit better, Things further off into the distance wouldn't be blurred to the same extent as the objects in the foreground.
So the clouds, ad possibly the hills (maybe a slight blur) would be clear like the helicopter. But the ship, that would be blurred like you have it.
Give it a try and see how it looks
