Where is the manpage of otools gcc etc. on macOS?
This answer from stackoverflow says that gcc is not installed by default on mac. Instead it 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. Alternatively, if you want help regarding gcc, you can use gcc --help