The reason that the gate is now closed is because the DND has reopened the range as of Aug/Sept 2019. I am friends with the Naval Chief who is running the Demolition course for new Bosuns in the navy. It was closed in 1988 due to a training incident (First link below for story) that killed 6 engineering students. The range was just reopened which probably explains the sudden closure of the gate. The second link below has a short video of a memorial they did just before they resuming training. I am uncertain about if/ when the gate will be open in the future, but i do believe that the course that was running during the time you got locked in is now over. Hope this helps you all out!