3.13.0-151.201) is not available, but a newer one is (3.13.0-166.216). We redirected you there.
/usr/share/help/gl/gnome-terminal/index.docbook is in gnome-terminal-data 3.6.2-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/help/gl/gnome-terminal/../../../help-langpack/gl/gnome-terminal/index.docbook