Grsecurity stable patches to be limited to sponsors

Um, no it's not. If I've modified some GPLed code, I don't have to give it to anyone at all, and I can choose to give it to a small chosen group without needing to give it to anyone else. I can't stop those people passing it on, but that's different.

If I use section 3b of the GPL and give them a written offer for the source code, then it's true that the offer must be valid for third parties. However, if I choose 3a and just givn the people to whom I distribute binaries the source code at the same time, then there's no obligation on me to distribute either the binaries or the source code to anyone else at all.