I am going to be the least agreeable on this.
So, the effect looks messy, it's all about one colour, you don't get to see proper details, it's no focal, no particular effect, no blending it's just that it's the same colour and seems like blendable.
Try to play with the colours, try to give the sig a depth, make a focal point and centralise all on the respective focal point, try o play with effects, make a proper brackground from brushes and stuff, add some fractals for nice effect, take car that the light on the focal when you put it won't be too like, balance everything so it looks like it comes from one piece.
Here is a good site for sigs.
You must be registered for see links
Or go on deviantart.com and you'll find some good tutorials for gimp as well.
Not trying to be bitchy, just advise you what others advised me as well.