We have incorporated the Colorbox lightbox feature for viewing images from the WordPress Gallery Shortcode (even the block gallery). You can now view your photos in a popup overlay viewer.
NOTE: We are planning to remove the lightbox in the next update because it does not work with actual image captions correctly (not compatible). We will be offering an alternative.
- Go to Appearance > Customize > Other Options
- Look for the Enable Gallery Lightbox setting and toggle the setting
- Click the Publish button
UPDATE: Because the Colorbox gallery script is now gone, the alternative is to check out a free plugin called WP Featherlight. This works with the block gallery. Of course, you also have the option to use other gallery plugins if you need something more robust.