I noticed in the Overview page, there is a "Site Status" section. This says "The site is opened!".

Is there a way to take the gallery offline into a 'maintenance' mode without just putting a password on the galleries?

Yes,if you see that message you have the site_upgrade plugin active and there is a button on the right named "Site » close". If you click it changes the message and the button text. There are three modes basically "open", "test mode" (loggind you can test the site) and "closed".

Thanks. I searched multiple times, and somehow completely missed this. I know I have seen it before.