If you source.list only contains the following standard sources for Debian Buster, there will be no man pages for your GCC compilers.
deb http://mirrors.163.com/debian/ buster main contrib non-free
deb-src http://mirrors.163.com/debian/ buster main contrib non-free
deb http://security.debian.org/debian-security/ buster/updates main contrib
deb-src http://security.debian.org/debian-security/ buster/updates main contrib
deb http://mirrors.163.com/debian/ buster-updates main contrib non-free
deb-src http://mirrors.163.com/debian/ buster-updates main contrib non-free
To install the documentation for GCC, you need to add backports to your source.list as below, where the contrib category contains the package gcc-doc and the non-free category contains the package gcc-8-doc.
deb http://mirrors.163.com/debian/ buster-backports main contrib non-free
The meaning of backports can be referred to here.
Backports are recompiled packages from testing (mostly) and unstable (in a few cases only, e.g. security updates) in a stable environment so that they will run without new libraries (whenever it is possible) on a Debian stable distribution.
Backports cannot be tested as extensively as Debian stable, and backports are provided on an as-is basis, with risk of incompatibilities with other components in Debian stable. Use with care!