GIMP actually has a pretty good tool/filter for stuff like this, called "Selective Gaussian blur". Depending on the image you're editing, you'll have to play around with it a little to get the right effect, however it's faster than using the smudge tool, in my opinion.
Here's an example: The first image has a ton of banding (or whatever it's called), while, with the second image, I've just used selective Gaussian blur across the whole image (which took 10 seconds) to make it look much more smooth: