I've set the fixed width and page height on the master as well as the page properties and still when I zoom out on a mobile my gallery page looks like this.
I want it to be able to zoom in, but when a user zooms out to restrict it to the outer edges of the photo.
It works for the desktop layout perfectly. Any answers are much appreciated! Thanks!
I have noticed that for a separate fixed width (phone) layout you have selected fluid width and this seems to be the root cause of your issue.
I suggest you change the same to the fixed width and see if your issue resolves.