Since I smudge hair, I'ma tell you how I do so. I also use gimp, so what I use will most likely be able to be used by you.
So, I take the smudge tool and use a soft brush(not too soft, just a bit softer than the hard brush) and set the size to 2 for manes and 3 for tails. I keep the force at 50 or lower, only make it higher if I really have to fight with the mane(but that would make so your basically redrawing the mane) and have the strength at 96. Make sure to have curves so the hair doesn't look like someone straightened it.
Sunny
