But, as others have mentioned, in order to clean up your lines, use a pen tool. In GiMP, which I use as well, there is the Paths Tool. You can use this to curve and to trace lines. A note about the lines, a huge note: Don't make the lines of the clothing bigger than the lines of the base. It looks off and can be easily rectified by selecting a thinner stroking line.
Outlines are always a good thing to have- it gives it a more clean appearance.
In terms of the clothing, very cute. But you don't have to list credits on the image.
Hyperster has many bases in the .psd format- these make it very easy to change skin color, eye color, etc, when you lock the layer's transparency. Try utilizing this function next time. (need help? Note me.)
The design is really nice! especially those shoes!! Tip - make the top and bottom outline thiner and add outline to the hair, but since you use gimp, which don't have a curve tool i think that's okay! but like I said, NICE!!