If that case were decided the other way, it would mean that government had a law that restricting their right to free speech. You will notice that they did not have the right to speak at the funeral, because the private entity running the funeral is not obligated to give them freedom of speech.

It is true that there are exceptions to the "congress shall make no laws ..." part of the first amendment; so by analogy, there might be some exceptions to the GPL's "no further restrictions" clause, but you have not explained why grsecurity falls into any of those hypothetical exceptions.