/usr/share/help/ru/gdm/index.docbook is in gdm3 3.28.0-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/help/ru/gdm/../../../help-langpack/ru/gdm/index.docbook
/usr/share/help/ru/gdm/../../../help-langpack/ru/gdm/index.docbook