/usr/lib/ruby/ is in ruby-gdk3 3.1.0-1+deb9u1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/
/usr/lib/ruby/vendor_ruby/gdk3/
/usr/lib/ruby/vendor_ruby/gdk3.rb
/usr/lib/ruby/vendor_ruby/gdk3/atom.rb
/usr/lib/ruby/vendor_ruby/gdk3/cairo.rb
/usr/lib/ruby/vendor_ruby/gdk3/color.rb
/usr/lib/ruby/vendor_ruby/gdk3/cursor.rb
/usr/lib/ruby/vendor_ruby/gdk3/deprecated.rb
/usr/lib/ruby/vendor_ruby/gdk3/event.rb
/usr/lib/ruby/vendor_ruby/gdk3/loader.rb
/usr/lib/ruby/vendor_ruby/gdk3/rectangle.rb
/usr/lib/ruby/vendor_ruby/gdk3/rgba.rb
/usr/lib/ruby/vendor_ruby/gdk3/screen.rb
/usr/lib/ruby/vendor_ruby/gdk3/window-attr.rb
/usr/lib/ruby/vendor_ruby/gdk3/window.rb