A gallery is the collection of products on the wall within the Designer.
To see information about the gallery, click once on any product in the gallery.
You'll notice that all products in the gallery now have a red border, and the panels on the left of your screen were replaced with information about the gallery:
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There are three actions you may take with a gallery, using the buttons at the bottom left:
Rotates the contents of the gallery 90 degrees. Be aware that as this changes images from vertical to landscape the crop that you had applied will be lost. You can also type "o" to rotate the gallery.
Love the gallery you have, but need to invert it? This action will mirror the gallery so it is horizontally inverted. This does not change the layout of the products so the crop that you applied to the products will be preserved. You can also type "m" to mirror the gallery.
If you want to manually clear the gallery so you can add a product, this button will remove it for you. Typing your delete button is the keyboard shortcut for deleting the entire gallery as well.