/usr/lib/ruby/gems/ is in libruby2.3 2.3.3-1+deb9u2.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/gems/2.3.0/
/usr/lib/ruby/gems/2.3.0/build_info/
/usr/lib/ruby/gems/2.3.0/cache/
/usr/lib/ruby/gems/2.3.0/doc/
/usr/lib/ruby/gems/2.3.0/extensions/
/usr/lib/ruby/gems/2.3.0/gems/
/usr/lib/ruby/gems/2.3.0/gems/rdoc-4.2.1/
/usr/lib/ruby/gems/2.3.0/gems/rdoc-4.2.1/bin/
/usr/lib/ruby/gems/2.3.0/gems/rdoc-4.2.1/bin/rdoc
/usr/lib/ruby/gems/2.3.0/gems/rdoc-4.2.1/bin/ri
/usr/lib/ruby/gems/2.3.0/specifications/
/usr/lib/ruby/gems/2.3.0/specifications/default/
/usr/lib/ruby/gems/2.3.0/specifications/default/bigdecimal-1.2.8.gemspec
/usr/lib/ruby/gems/2.3.0/specifications/default/io-console-0.4.5.gemspec
/usr/lib/ruby/gems/2.3.0/specifications/default/json-1.8.3.gemspec
/usr/lib/ruby/gems/2.3.0/specifications/default/psych-2.1.0.gemspec
/usr/lib/ruby/gems/2.3.0/specifications/default/rdoc-4.2.1.gemspec