I'm sorry I'm not very familiar with GIMP. However, when saving anything as a gif, you will lose a lot of quality.
For example, this sig I made for Jupiter this week.
This is a frame saved as a .png. It looks so nice and smooth and pretty right? However .png cannot be saved as an animated file...boo.
And here is the sig saved as a .gif (and animated!)
If you look closely you will see there is a bit of "grain" to it. There is unfortunately no way of truly avoiding that.
However! I'm not sure about GIMP's settings, but make sure when you save hte gif, that you set it to the highest amount of colors posssible (should be 256). Then play around with the settings to get what you want. Maybe someone else can be more specific but thats generally what i do with this kind of stuff...
Composition wise, I think you have a lovely signature! Though, the animation is a bit too flashy...it might give someone seizure if they are epileptic. I'm not sure what you are working with but it seems you should try doing more frames than just the few you have here. ^^ I use KMPlayer and the Capture Batch option to get a lot of screenshots quickly for my gifs. I hope that helps!