It all comes down to the definition of "free". Debian has the Debian Free Software Guidelines, upon which the OSI Open Source Definition is based. If you read

It all comes down to the definition of "free". Debian has the Debian Free Software Guidelines, upon which the OSI Open Source Definition is based. If you read them, you'll see that the GFDL with invariant sections fails the clause regarding "modification and derived works". Because it explicitly restricts your ability to modify and distribute. Trisquel may have a different interpretation. I understand they are closely aligned with the FSF, so may not agree with the Debian stance. This is one point upon which I do think the FSF is entirely wrong. Putting invariant sections into the GFDL made the licence firstly overly complex, and secondly made it incompatible with the OSI and DFSG interpretation of freedom. All just so they could include immutable political content in their documentation. The fallout from this made it a very poor choice to use for documentation, and was a spectacular own goal. If you follow the licence to the letter, it means you can't embed source code examples without being required to also include the invariant sections (you aren't permitted to delete them). This makes the licence impractical to comply with for a lot of common use cases. Overall, it's simpler to ignore the licence entirely and simply use the same licence as used by the rest of a given project's codebase. I did use the GFDL in a few projects at the time of its creation, but no longer do so after getting a better understanding of it. It's a bad licence.