Installing GCC Man-Pages on Debian

Without the non-free repository you will get the following error:
$ sudo apt install gcc-doc
[sudo] password for kkr:
Reading package lists... Done
Building dependency tree      
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 gcc-doc : Depends: gcc-6-doc (>= 6.1.0-1~) but it is not installable
E: Unable to correct problems, you have held broken packages.
To solve this problem, you need this line in your /etc/apt/sources.list:
deb stable non-free
Tested on Debian Stretch.