Well you kinda already stated the difference, Blender is for 3D while GIMP is more for 2D, basically it acts as a watered down Photoshop(not that it's a bad program but it is free for a reason). You can use them together by modeling in Blender and then Painting UV maps in GIMP, though they are two different types of programs.
So if you want to make signatures that are 2D digital paintings or vector art, then I'd use GIMP. Though, if you plan on modeling and animating in 3D then I'd use Blender. They both have potential for graphic design for their own mediums, it's hard to tell you which you will need, without know what you want to do with them.