/usr/share/gtk-doc/html/gdk3 is in libgtk-3-doc 3.18.9-1ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/gtk-doc/html/../../doc/libgtk-3-doc/gdk3
/usr/share/gtk-doc/html/../../doc/libgtk-3-doc/gdk3