According to man man, you can optionally give the section of the manual before the page. Section 1 is user commands, 2 system calls and 3 library functions, so:

man 1 printf
man 3 printf

give the shell command and the C library function printf, respectively.

The manpages are provided in contrib packages, gcc-doc etc. (See the links at the top-right of the linked page for all the releases where the package is available.)

Debian 10’s default compiler is GCC 8. The GCC 8 documentation wasn’t packaged in time for Debian 10’s release, but it is available in backports, along with the corresponding gcc-doc package. To install it, you need to enable backports with contrib and non-free, and install it from there explicitly:

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

Note though, even with gcc-doc you may want to install manpages-posix-dev for access to POSIX docs on ISO C standard library docs.

You can also get information beside man gcc (you have to change the order of your cmd) by using

info gcc

To get a short command description type

gcc --help

For a full manual install the gcc documentation by

gccv=$(gcc --version | awk '/gcc /{print $NF}' | cut -c 1)
sudo apt install gcc-$gccv-doc

and then type

xdg-open /usr/share/doc/gcc-$gccv-doc/gcc.html

to read the gcc manual in your browser.

gcc isn't installed anymore by Xcode, it really installs clang and calls it gcc

usxxplayegm1:~ grady$ which gcc
/usr/bin/gcc

usxxplayegm1:~ grady$ /usr/bin/gcc --version

Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.0.2
Thread model: posix

you need man clang

I thought it was a symlink, but ls -l doesn't list it as a symlink, so either it is a hard link or some other sort of trickery.

The documentation isn't in the "gcc" package, it's in a separate package called "gcc-doc".