Its hard to do vector sigs, in my opinion i think it was pretty well done, with a few exceptional changes i think you should put it as your sig.
1. Change the light source to the lower left hand corner.
2. make the text a little more subtle.
3. Apply image, Gaussian blur to a 3.0 pixel and mess around with opacity and blending options.
4. if gimp can use clipping masks, try using a couple vector clipping masks, if you dont know what a clipping mask is ill be more then willing to help, i just got into them and man do they make sig making faster and easier.