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.