A heads-up on this gate situation which caused a lot of anxiety for the passengers. Gate D23 is between D21 (a normal looking gate area) and a cul-de-sac of gates D24-27. Although there is a sign that says "D23" there is no apparent gate. There is a large door that is cordoned off and marked as an "emergency exit" and an elevator. Here's how the system worked for us (although our plane was delayed and that increased the confusion and chaos):
At boarding time, people who needed to pre-board were taken up in the elevator. If anyone tried to take the elevator before then, the buttons inside didn't work and they looked pretty silly as they exited the elevator. After pre-boarding was finished, the "emergency exit" door was opened and everyone else entered and climbed a set of stairs to enter the jet bridge.
It would have been so nice to have a sign, an announcement, or an employee there to explain the procedure but that wasn't the case. Hopefully this will help others in the future.