There are 2 distinct methods for hosting the data and images for image galleries:
Hosting the images locally is simpler than using a Google Sheet. The Google Sheet method gives you more flexibility including customized image galleries for different users.
Click on Add Gallery button in the upper right corner of the Image Gallery page. Assign a unique name to your new image gallery.
If you choose to modify the Image Gallery, you will see all of the images contained in the Image Gallery. You can dictate the order of the images by clicking the up and/or down arrows next to each image. You will also be able to view the low-res and high-res sizes of each image contained in the Image Gallery.
Click on Add Gallery button in the upper right corner of the Image Gallery page. Assign a unique name to your new image gallery. Check the chechbox From Google Sheet and then paste the ID of the Google Sheet containing the data about the images.
If you need to modify any data for an image gallery linked to a Google SHeet, you will edit the data using Google Sheets and not in the Designer server interface.