Did you... talk to Harland? Do you remember what he said?
He's not opening the gate, because the passages are filled with Lurkers and I'm assuming he'd rather not get stabbed.
Did you...speak to blaine?
I cleared those tunnels near the start of the game. I even do a little dance infront of the security camera harland uses to watch said shortcut. Showing him its safe everytime I see blaine, who btw, would be SO much easier to visit if I could just, use, that, shortcut.
Again seems like a forgotten feature more than anything since the framework is there, its simply missing the dialogue and or function to open said door.