Create a new gallery and select the Product Gallery from the Gallery Settings dropdown.

Responsive Image Gallery
Image Viewer
Justified Gallery
Masonry Image Gallery
Simple Portfolio
Single Thumbnail Gallery
Carousel
Polaroid PRO
Grid PRO
Slider PRO
Product Gallery

Click on the "Ecommerce" and then click on "Advanced".

Click the "Transfer Attachment Details" field, followed by the "Select Master Product" option. 

Click the "Transfer Attachment Details" field.

A new panel will open up where you can click on "Generate Master Product". A product will be generated. Select this and click on "Select Master Product".

A new panel will open up where you can "Generate Master Product".

Don't forget to save, publish or update your gallery.

Go back to your Master Product and Click "edit". This will take you to the product in WooCommerce.

Open up the Generated Master product and go to the "FooGallery" tab. 

Click the "Enable Downloads" field. (Without this enabled, downloads won't work.)

Next, go to the "Variations" tab. Here you can edit the image sizes: you can add a download height and width for each size (leave the large size blank to allow for the full-size image download), and add prices.

How to Create an Ecommerce Gallery with Download Options