user_5@zrl5:~$ /etc/init.d/gdm restart
Rather than invoking init scripts through /etc/init.d, use the service(8)
utility, e.g. service gdm restart

Since the script you are attempting to invoke has been converted to an
Upstart job, you may also use the restart(8) utility, e.g. restart gdm
restart: Rejected send message, 1 matched rules; type="method_call", sender=":1.129" (uid=1001 pid=2884 comm="restart) interface="com.ubuntu.Upstart0_6.Job" member="Restart" error name="(unset)" requested_reply=0 destination="com.ubuntu.Upstart" (uid=0 pid=1 comm="/sbin/init"))
user_5@zrl5:~$ sudo /etc/init.d/gdm restart