Include all post galleries in the lightbox.

gistfile1.txt

add_action( 'wp_footer', function() {

    wp_enqueue_script( 'jquery' );

    ?>

    <script>
        document.addEventListener( 'click', function( event ) {

            var target = event.target.closest( '.gallery-item a' );

            if ( ! target ) {
                return // not a gallery item
            }

            // The current link we are clicking on.
            var currentLink = target.getAttribute( 'href' );

            if ( ! currentLink ) {
                return; // item doesn't have a link.
            }

            // Note: you could add a complex check here as well to ensure the gallery item is linking to an image
            // and not an attachment page.

            // Prevent click event since we want to display lightbox.
            event.preventDefault();

            // Get parent post container.
            var parent = target.closest( '#content' );

            if ( ! parent ) {
                return; // not a gallery inside a post
            }

            // Define array for gallery items.
            var galleryItems = [];

            // Define active index to ensure the item we are clicking on is the item shown in the lightbox.
            var activeIndex = 0;

            // Loop through all post gallery images and save in an array.
            document.querySelectorAll( '.gallery-item a' ).forEach( function( galleryItemLink, index ) {

                var link = galleryItemLink.getAttribute( 'href' );

                if ( link ) {

                    galleryItems.push( {
                        src: link,
                        opts: { thumb: link }
                    } );
                }

                if ( link === currentLink ) {
                    activeIndex = index;
                }

            } );

            // Trigger lightbox.
            if ( galleryItems
                && 'object' === typeof wpex_fancybox_params
                && 'function' === typeof jQuery
                && 'undefined' !== typeof jQuery.fancybox
            ) {

                jQuery.fancybox.open( galleryItems, wpex_fancybox_params, activeIndex );
            }

        } );

    </script>

<?php });