The terms “free software” and “open source”, while describing almost the same category of software, stand for views based on fundamentally different values. The free software movement is idealistic, and raises issues of freedom, ethics, principle and what makes for a good society. The term open source, initiated in 1998, is associated with a philosophy which studiously avoids such questions. For a detailed explanation, see https://www.gnu.org/philosophy/open-source-misses-the-point.html.
The GNU Project is aligned with the free software movement. This doesn’t mean that all GNU contributors and maintainers have to agree; your views on these issues are up to you, and you’re entitled to express them when speaking for yourself.
However, due to the much greater publicity that the term “open source” receives, the GNU Project needs to overcome a widespread mistaken impression that GNU is and always was an “open source” activity. For this reason, please use the term “free software”, not “open source” or “FOSS”, in GNU software releases, GNU documentation, and announcements and articles that you publish in your role as the maintainer of a GNU package. A reference to the URL given above, to explain the difference, is a useful thing to include as well.