Note: This feature is only available on the Pro and Studio plans.
How to make a template gallery
Go to Settings > Galleries & Designs > Sales Galleries > Sales Gallery Templates to make a new sales gallery template. You can edit a sales gallery template as if it were a normal gallery. The only thing you can't do is upload photos.
How to apply a template gallery
In the "Add Gallery" modal (when you are creating a gallery), click the "Copy from Template" button, and then choose a template from the dropdown list. This will copy all the settings in the template gallery into the new gallery you're creating.