You will need Apple's C/C++ compiler to install new versions of Gosu or any other C-based RubyGems, like Texplay, EventMachine etc. It doesn't have to be Xcode, you can also use Apple's command line toolchain:
http://apple.stackexchange.com/a/106716If the current version of Gosu is all you need in the foreseeable future, you don't need to reinstall anything, though.
The same applies to SDL 2, which you've probably installed using Homebrew.