/usr/share/gtk-doc/html/libgksu is in libgksu2-dev 2.0.13~pre1-8.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/gtk-doc/html/../../doc/libgksu2-dev/html
/usr/share/gtk-doc/html/../../doc/libgksu2-dev/html