One is not under obligation to guarantee that new versions are distributed to someone, which also means obligations can be terminated for any reason. So while grsecurity might not be doing the morally and ethically right thing, I do not think they are violating the GNU GPL. You're still free to redistribute the patches, but grsecurity isn't under obligation to give you future updates.
Their agreement text is located at https://grsecurity.net/agree/agreement_faq
