/usr/lib/x86_64-linux-gnu/ruby/ is in ruby-gnome2-dev 3.2.4-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/glib-enum-types.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rb-gobject-introspection.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rb-pango-conversions.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rb-pango.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbgcompat.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbgdk.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbgdkconversions.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbglib.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbglib2conversions.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbglibdeprecated.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbgobject.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbgtk.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbgtkconversions.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbgtkmacros.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbgutil.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbgutil_list.h
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/rbgutildeprecated.h