The text of the GPL does not follow the DFSG; license texts are not expected to. This is nothing new.
In the GNU emacs manual, the gnu manifesto is invariant. Suppose that we change the license as follow. We invent a "GNU emacs documentation license" as follow:
Preamble <insert here the GNU manifesto>1. some lisense text describing that you can freely modified and distribute the document under the same lisence.
Do you think that such a license is free?If yes; I do not see any real difference with the FDL which make the GNU manifesto as an invariant section
If not; then also the GPL would be not free because of it's preamble. Olive