問題：

$ man gcc
No manual entry for gcc

看到這個方法，但是失敗了

$ sudo apt install gcc-doc
Reading package lists... Done
Building dependency tree... Done
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-10-doc (>= 10.1.0-1~) but it is not installable
E: Unable to correct problems, you have held broken packages.

看到這個方法，解決了問題

echo deb http://deb.debian.org/debian buster-backports main contrib non-free | sudo tee /etc/apt/sources.list.d/buster-backports.list
sudo apt update
sudo apt install -t buster-backports gcc-doc