How to fix "No manual entry for gcc"?

Look at how you set MANPATH in your .profile. Instead of, e.g.,

export MANPATH=/opt/local/man:/usr/local/man

you should do

export MANPATH="/opt/local/man:/usr/local/man:$MANPATH"

And then (breathe) open a new Terminal window.

This way your .profile isn't wiping out (or reflecting an old version of) the system's way of setting MANPATH, which is modified when you install (or reinstall) Xcode.

This may be old and doesn't quite answers your concrete question, but I found myself in the same problem with Kali Linux and I solved it with this command, maybe is useful for someone:

apt install gcc-doc

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.