/usr/share/gtk-doc/html/gnutls is in gnutls-doc 3.5.8-5+deb9u4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/gtk-doc/html/../../doc/gnutls-doc/api-reference
/usr/share/gtk-doc/html/../../doc/gnutls-doc/api-reference