Well it's IRIX, so you'd just create a little bourne shell script in /etc/init.d, with symlinks from e.g. /etc/rc2.d to start/kill it on the respective run levels.

This was a pretty critical system though so why not just place this particular program inside /etc/inittab ?

Before systemd:

  sudo service gates restart

...and after too, it still exists and maps to systemctl on ones that use systemd.