Inside the elevator is another panel you need to hack to get it moving. At the top, hop through the ceiling and go outside, there will be enemies waiting to ambush you (don't take too long because they'll open the door). While the glass takedown looks great, I'd go with taking down one of the two stun baton guards because it will leave you in a better position. Just get moving around and use takedowns on the stun and armored guards and this should be a fast fight. Off to one side of the elevator is another panel to open the gate. Hack it and climb through the door. Slide around to the back of the building and from the antenna, grapple up above.
Once you reach the landing, turn left and jump to the pipe on the side of the building. Follow that around as far as you can, and then you can grapple strait up and then you'll want to grapple again off to the side. Head back along the building and up the ladder into the elevator shaft. Run and Jump across to the far side landing (down one level). Head forward and take a right and you'll come back outside to a giant antenna again. Walk out and then tightrope walk on the wire. From about halfway along the wire, you'll be able to grapple up on top of the caged in walk way you just come from and from there you can grapple strait up above you.
You'll be just outside of the elevator shaft now and in the back of the room, to the left will be another panel to hack. Hack that and it will open a door far above you. You have to be right under the door to reach it with the grapnel. Once up there, go through the hallway until you come to a dead end. On the right side a little ways up you will find an air vent. You want to take the lower path in the vent which will lead you outside. From here, you can get to the gargoyles.