gmp-doc on Debian Linux 13.6

I wanted to test compiling pari 2.8.1 from source code.

Operating System : Debian Linux 13.6 (trixie) for AMD64.

The hardware is physical AMD Ryzen processor (CPU).

I installed "build-essential" (which is for building Debian packages, see the Debian Policy Manual

https://www.debian.org/doc/debian-policy/index

https://www.debian.org/doc/debian-policy/ch-source.html

an informational list can be found in /usr/share/doc/build-essential/list

In my case the "build-essential" was just to get GNU make 4.4.1 and GCC 14.2 for Debian trixie as the additional dpkg-dev for building Debian packages was not really needed just to compile PARI from source (I was not building a PARI Debian package).

Also, I installed in addition to "build-essential", extra packages such as GNU GMP 6.3 and ncurses and readline 8.2 using the commands:

apt install libreadline-dev
apt install libncurses-dev
apt install libgmp-dev  (I have a question on this command, see below)

Then I configured PARI using

 ./Configure --prefix=$HOME/pari --with-gmp -with-readline --mt=pthread 

This compiled fine using "make" and "make install".

The resulting PARI gp executable in $HOME/pari/bin/gp reports that it is compiled with readline and GMP support using threading engine pthreads and reports number of nbthreads.

So the compilation from source code was a success.

During installing of GNU GMP I observed something strange, namely that gmp-doc (the documentation for GNU GMP ?) could not be installed.

Is this a misunderstanding from my part, please, or do I see at :

https://packages.debian.org/search?keywords=gmp-doc

that gmp-doc is in a "non-free" source of Debian Linux.  Why would that be the case ?

Is the format of the documentation of GNU GMP nonfree or the contents ?

The filelist seems to be available at https://packages.debian.org/trixie/all/gmp-doc/filelist

The Policy Manual states : 

.2.4. The non-free archive area

The non-free archive area contains supplemental packages intended to work with the Debian distribution that do not comply with the DFSG or have other problems that make their distribution problematic. They may not comply with all of the policy requirements in this manual due to restrictions on modifications or other limitations.

I tried seeing in the Debian bugs database why the distribution of gmp-doc would be problematic or why would gmp-doc not comply with DFSG ??

Regards,
David Stes