XCode includes the gcc customized for MacOS, but it is incompatible with a lot of wonderful softwares.

Run 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

or

brew install https://raw.github.com/adamv/homebrew-alt/master/duplicates/apple-gcc42.rb

It just works.

Reference:

https://github.com/mxcl/homebrew/wiki/Custom-GCC-and-cross-compilers