It sounds like you're trying to do one of two things: stop people from patenting your idea, or stop people from building on your idea and patenting that. In neither case is any copyright license (including the GPL) likely to be of much help.
If all you want is to stop people from patenting your idea, you don't need to worry about licensing. One requirement for a patent is that the invention be new; if someone else has already shared the idea with the world before you apply for a patent, you can't get one.
On the other hand, if you want to stop people from patenting things derived from your idea, the GPL is not going to do anything to help you. The GPL is a copyright license. It authorizes licensees to exercise the exclusive rights of copyright: reproduction, distribution of copies, creation of derivatives, public display, and public performance. That's all it does.
Not everything is copyrightable. Functional aspects of something are not copyrightable; anything that'd be covered by a utility patent isn't going to be protected by copyright. Copyright protects creative works. Copyright might protect the specification you write, but only to the extent that there's creativity involved in actually writing the spec. It offers no protection to the system you're specifying. It protects the expression of ideas, not the ideas themselves.
If someone reads your spec, thinks of an improvement to your system, and describes their improvement (either referencing your spec or rephrasing it), they haven't done anything they need your permission for. You don't need a copyright license to read a specification or to make use of the ideas in it; "creating a derivative work" doesn't include cases where their thing is derivative of your ideas.
So what does the GPL patent clause mean? It doesn't mean "if you come up with your own thing you must not patent it." It means "if you redistribute the covered work or a derivative of it, you must license any relevant patents." If someone comes up with an idea for an improvement and writes up their own spec for that idea, they can share it with the world without worrying about your copyright (because copyrights don't cover ideas) or the GPL. The only way to stop this is to try to come up with a "patentleft" concept, but patents really aren't well-suited to this concept.