これで。
# gem environment
インストール場所だけでなく、他にもいろいろ出てきます。
RubyGems Environment: - RUBYGEMS VERSION: 2.5.1 - RUBY VERSION: 2.3.1 (2016-04-26 patchlevel 112) [x86_64-linux-gnu] - INSTALLATION DIRECTORY: /var/lib/gems/2.3.0 - USER INSTALLATION DIRECTORY: /root/.gem/ruby/2.3.0 - RUBY EXECUTABLE: /usr/bin/ruby2.3 - EXECUTABLE DIRECTORY: /usr/local/bin - SPEC CACHE DIRECTORY: /root/.gem/specs - SYSTEM CONFIGURATION DIRECTORY: /etc - RUBYGEMS PLATFORMS: - ruby - x86_64-linux - GEM PATHS: - /var/lib/gems/2.3.0 - /root/.gem/ruby/2.3.0 - /usr/lib/x86_64-linux-gnu/rubygems-integration/2.3.0 - /usr/share/rubygems-integration/2.3.0 - /usr/share/rubygems-integration/all - GEM CONFIGURATION: - :update_sources => true - :verbose => true - :backtrace => false - :bulk_threshold => 1000 - REMOTE SOURCES: - https://rubygems.org/ - SHELL PATH: - /usr/local/sbin - /usr/local/bin - /usr/sbin - /usr/bin - /sbin - /bin - /usr/games - /usr/local/games - /snap/bin