gemファイルのインストール場所を調べたりする方法

$ gem environment を使うとOK

ironsand@osx gem environment
RubyGems Environment:
  - RUBYGEMS VERSION: 2.0.3
  - RUBY VERSION: 2.0.0 (2013-06-27 patchlevel 247) [x86_64-darwin11.4.2]
  - INSTALLATION DIRECTORY: /Users/ironsand/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0
  - RUBY EXECUTABLE: /Users/ironsand/.rbenv/versions/2.0.0-p247/bin/ruby
  - EXECUTABLE DIRECTORY: /Users/ironsand/.rbenv/versions/2.0.0-p247/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86_64-darwin-11
  - GEM PATHS:
     - /Users/ironsand/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0
     - /Users/ironsand/.gem/ruby/2.0.0
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :backtrace => false
     - :bulk_threshold => 1000
     - "install" => "--no-ri --no-rdoc"
     - "update" => "--no-ri --no-rdoc"
  - REMOTE SOURCES:
     - https://rubygems.org/

あと $ gem help commands すると色々教えてもらえて便利！

参考

ruby - How can I find where gem files are installed? - Stack Overflow
http://stackoverflow.com/questions/19072070/how-can-i-find-where-gem-files-are-installed/19072136?noredirect=1#19072136