The GNU project has a list of free GNU/Linux distributions. But it lists only a few Linux distros, although hundreds do exist. Even more, very common distributions like Debian, Linux Mint or Gentoo aren’t listed.
Why is the list so restricted?
They only list GNU/Linux distributions that follow the GNU FSDG (Free System Distribution Guidelines).
That the software (as well as the documentation, fonts etc.) is licensed under an appropriate FSF-approved license is one condition, but it’s not the only one. That’s why even a GNU/Linux distribution that only ships with free/libre software/information might not get listed.
Another guideline, as example: The distribution is not allowed to "encourage" users to obtain "any nonfree information for practical use" (which includes software).
In practice this could mean that the distribution project’s server may not host a repository that contains proprietary software (or free/libre software that downloads proprietary software from somewhere else). (This is the reason why Debian is not listed.)
At the bottom of the page you linked is a link named "why we don't endorse some common distributions"
There you will find this text:
We're often asked why we don't endorse a particular system—usually a popular GNU/Linux distribution. The short answer to that question is that they don't follow the free system distribution guidelines. But since it isn't always obvious how a particular system fails to follow the guidelines, this list gives more information about the problems of certain well-known nonfree system distros.
To learn more about the GNU/Linux systems that we do endorse, check out our list of free GNU/Linux distributions.
Except where noted, all of the distributions listed on this page fail to follow the guidelines in at least two important ways:
They do not have a policy of only including free software, and removing nonfree software if it is discovered. Most of them have no clear policy on what software they'll accept or reject at all. The distributions that do have a policy unfortunately aren't strict enough, as explained below.
The kernel that they distribute (in most cases, Linux) includes “blobs”: pieces of object code distributed without source, usually firmware to run some device.
In other words to be listed under GNU the distro must shun all non-free software. This seems to include not telling the user how to install non-free software on the distro if you examine some of the specific examples.