/usr/lib/ruby/gems/ is in libruby2.1 2.1.5-2+deb8u3.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/gems/2.1.0/
/usr/lib/ruby/gems/2.1.0/build_info/
/usr/lib/ruby/gems/2.1.0/cache/
/usr/lib/ruby/gems/2.1.0/doc/
/usr/lib/ruby/gems/2.1.0/extensions/
/usr/lib/ruby/gems/2.1.0/gems/
/usr/lib/ruby/gems/2.1.0/gems/rake-10.1.0/
/usr/lib/ruby/gems/2.1.0/gems/rake-10.1.0/bin/
/usr/lib/ruby/gems/2.1.0/gems/rake-10.1.0/bin/rake
/usr/lib/ruby/gems/2.1.0/gems/rdoc-4.1.0/
/usr/lib/ruby/gems/2.1.0/gems/rdoc-4.1.0/bin/
/usr/lib/ruby/gems/2.1.0/gems/rdoc-4.1.0/bin/rdoc
/usr/lib/ruby/gems/2.1.0/gems/rdoc-4.1.0/bin/ri
/usr/lib/ruby/gems/2.1.0/gems/test-unit-2.1.5.0/
/usr/lib/ruby/gems/2.1.0/gems/test-unit-2.1.5.0/bin/
/usr/lib/ruby/gems/2.1.0/gems/test-unit-2.1.5.0/bin/testrb
/usr/lib/ruby/gems/2.1.0/specifications/
/usr/lib/ruby/gems/2.1.0/specifications/default/
/usr/lib/ruby/gems/2.1.0/specifications/default/bigdecimal-1.2.4.gemspec
/usr/lib/ruby/gems/2.1.0/specifications/default/io-console-0.4.2.gemspec
/usr/lib/ruby/gems/2.1.0/specifications/default/json-1.8.1.gemspec
/usr/lib/ruby/gems/2.1.0/specifications/default/minitest-4.7.5.gemspec
/usr/lib/ruby/gems/2.1.0/specifications/default/psych-2.0.5.gemspec
/usr/lib/ruby/gems/2.1.0/specifications/default/rake-10.1.0.gemspec
/usr/lib/ruby/gems/2.1.0/specifications/default/rdoc-4.1.0.gemspec
/usr/lib/ruby/gems/2.1.0/specifications/default/test-unit-2.1.5.0.gemspec