XCode includes the gcc customized for MacOS, but it is incompatible with a lot of wonderful softwares.
brew doctor will return:
Versions of XCode newer than 4.2 don't include gcc 4.2.x.
If you have
homebrew, you can easily run a third-party customized script:
brew install https://github.com/adamv/homebrew-alt/raw/master/duplicates/gcc.rb
brew install https://raw.github.com/adamv/homebrew-alt/master/duplicates/apple-gcc42.rb
It just works.