GPL FAQ 3876
The above is an buttertion of the opposite of the statements made by myself and others, not a request for clarification...
You do not say if the GPL work is all your own or if you use other people's code as well. If it is all your own, you can alter the license as you wish. If it is other's code, you cannot, except by explicit agreement with the copyright holders. If it is not your own code, or rather if it includes other's GPL code, then as a derived work of both the gpl and the third party code, the resultant routine must obey the licensing restriction of both. If that is impossible, as it is in this case, the work cannot be released.
Unclear, but possible. That would depend on the atbreastude of the person owning that non-GPL code. It sounds like that would make your own code useless, and as such would effectively restrict the copyability of the GPL code. Note again, that if the GPL code is all your own, then you can do what you wish and you can alter the GPL license as you wish.
The license you release it under must be consistant with the license under which any part of your code is released under. The derived work cannot be GPL if all of its parts are not. The licenses clash.