There was one GNU-compatible real-time kernel released under the GPLv2- RTLinux. However, the website's been down for some time now. I think the project's been abandoned. There is an FTP repository from which you can download the source for RTLinux (here: ftp://ftp.rtlinux-gpl.org/pub/rtlinux), but it requires a username and password. And I have no idea how to go about getting one. The repository hasn't even been updated since 2001, so I wouldn't recommend even trying this with a modern system. The world of GNU/Linux has changed a lot since 1998 (when RTLinux was written). The mailing list archives no longer exist, as do the people who once hosted the list, who have been taken over by spammers.
Basically, the project is dead. If you want a newer version of Linux-libre, then you can use jxself's repository. There are instructions on how to do this here: https://trisquel.info/en/wiki/update-linux-libre-kernel under 'Using jxself's repository'.
