As the topic asks, where is the gate switch?
After the enemy run out of the fire base, the gate closes behind them.
The gate seems to be indestructible. I repeatedly shoot it with heavy bolters, boltguns, plasma guns, and threw krak grenades to no effect. Then I tried power fists and power swords in melee. The gate still stands closed.
I ran my assault marines around the interior walls looking for the lever/switch and couldn't find it.