After checking with WP Engine, they are saying that "the external call to smushpro.wpmudev.org is taking 40,300 ms on it's own on /galleries/fire. That may be preventing items from loading until it finishes and causing the timeout."
This is what WP Engine sent me:
Can you please investigate why Smush is sending request to Smushing server every time the gallery is being loaded? Gallery load time drops instantly when I disable Smush.