Install GCC Man Pages on Debian 10 Buster
The Man Pages for GCC compiler are not included by default on Debian 10 Buster and must be added by installing the Backport package named gcc-doc. To complete this task, I recommend you should add the Backports sources with options main contrib to your source files /etc/apt/sources.list.d/ directory on a file named buster-backports.list. You can do this with this command:
echo deb http://deb.debian.org/debian buster-backports main contrib | sudo tee /etc/apt/sources.list.d/buster-backports.list
After this, run:
sudo apt update
sudo apt install gcc gcc-doc
Now try running this to corroborate proper installation of GCC's man pages.
Note: You could also add the non-free option to your Backports source if you are interested in other unrelated non-free software later on, but it is not required for installing gcc-doc.