There are so many reasons for adding a Gallery to your website. If you are an artist or photographer the reasons are obvious. But you might also be showcasing products. If you are a bakery, for example, you might have a gallery of all the impressive, mouth watering cakes you've created. If you are selling clothing you with have images of women wearing your clothes. If you are a pre-school you might have images of happy children.
Whatever your gallery is about, the key to organzing is naming your images. The image names will be used on your site and will help Google love and find your website.
Don't Forget! The best way to complete the blocks is to open them and take note of what is needed. Then gather the information and go back to the form and upload it. You cannot save the form as you go, so have all your content on hand when you want to complete the block.