I am on a fresh install of OS X Mountain Lion. I have installed rails via:
sudo gem install rails
Everything seems to install correctly, but wh
Yes, OSX comes standard with a lot of great software for Ruby on Rails, as well as PHP, Mysql, etc. However, sometimes it's better for sustainment purposes to use a 3rd party installer to get everything you want without digging through your /usr/ directory.
I recommend checking out http://railsinstaller.org/
With one easy install, you have everything you could want for a Rails project, including common software people use, and the site even has a tutorial. I recommend going this way. It saves you time. Plus, it comes with an easy uninstaller that it will put in your Applications folder to remove if you're not happy with the configuration. Enjoy.