Open Ruby installed gems in editor
If you use the Ruby that comes with 10.9, gems are installed in /Library/Ruby/Gems/2.0.0/:

open /Library/Ruby/Gems/2.0.0/gems/plist-3.1.0/lib/plist/parser.rb


If you use a Ruby installed with Homebrew, gems are installed in /usr/local/lib/ruby/gems/*/gems/.