A gallery is the collection of products on the wall within the Designer.
To see information about the gallery, click and drag over all products in the gallery, or type Command + A.
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:
There are six actions you may take with a gallery, using the buttons at the bottom left:
Align Products
By clicking and dragging to select multiple products on the wall, you can align them to top, bottom, left or right. Click Finish Alignment when those selected products are aligned. If you need to align more products, now click and drag to select another set of products to align.
Keyboard shortcut: instead of clicking the Align Products button, just type Shift + Arrow (whichever arrow direction you want to align; so if you want to align top, type Shift + Up arrow) when you have products selected. I find this much faster especially when I'm aligning multiple sets of products.
Send to Back/Front
If you have products that are overlapping, you can click to send one product to the back or front.
Rotate Products
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.
Mirror Products
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.
Delete Products
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.