Custom Gallery Shortcode output

gallery.php

<?php

/**
 * Force the Gallery Shortcode attributes
 */
add_filter(
    'shortcode_atts_gallery',
    function ($out) {
        $out['size'] = 'medium';
        return $out;
    },
    11
);

add_filter('post_gallery', 'vralle_by_may_be_amp_gallery', 11, 2);
function vralle_by_may_be_amp_gallery($output, $attr)
{
    /**
     * Force Gallery Shortcode attributes for AMP
     * @link https://github.com/ampproject/amp-wp/blob/develop/includes/embeds/class-amp-gallery-embed.php
     */
    // remove_filter('post_gallery', 'vralle_by_may_be_amp_gallery', 11);
    // $attr['lightbox'] = true;
    // $attr['amp-carousel'] = false;
    // $output = gallery_shortcode($attr);
    // add_filter('post_gallery', 'vralle_by_may_be_amp_gallery', 11, 2);
    $output = vralle_by_gallery($attr);
    return $output;
}

function vralle_by_gallery($attr)
{
    $post = get_post();

    static $instance = 0;
    $instance++;

    $atts = shortcode_atts(
        array(
            'order'      => 'ASC',
            'orderby'    => 'menu_order ID',
            'include'    => '',
            'exclude'    => '',
            'id'         => $post ? $post->ID : 0,
            'size'       => 'medium',
            'link'       => 'large',
            'columns'    => 1,
        ),
        $attr,
        'gallery'
    );

    $id = intval($atts['id']);

    if (!empty($atts['include'])) {
        $_attachments = get_posts(
            array(
                'include' => $atts['include'],
                'post_status' => 'inherit',
                'post_type' => 'attachment',
                'post_mime_type' => 'image',
                'order' => $atts['order'],
                'orderby' => $atts['orderby']
            )
        );
        $attachments = array();
        foreach ($_attachments as $key => $val) {
            $attachments[$val->ID] = $_attachments[$key];
        }
    } elseif (!empty($atts['exclude'])) {
        $attachments = get_children(
            array(
                'post_parent' => $id,
                'exclude' => $atts['exclude'],
                'post_status' => 'inherit',
                'post_type' => 'attachment',
                'post_mime_type' => 'image',
                'order' => $atts['order'],
                'orderby' => $atts['orderby']
            )
        );
    } else {
        $attachments = get_children(
            array(
                'post_parent' => $id,
                'post_status' => 'inherit',
                'post_type' => 'attachment',
                'post_mime_type' => 'image',
                'order' => $atts['order'],
                'orderby' => $atts['orderby']
            )
        );
    }

    if (empty($attachments)) {
        return '';
    }

    if (is_feed()) {
        $output = "\n";
        foreach ($attachments as $att_id => $attachment) {
            $output .= wp_get_attachment_link($att_id, $atts['size'], true) . "\n";
        }
        return $output;
    }

    $columns = intval($atts['columns']);
    $selector = "gallery-{$instance}";
    $size_class = sanitize_html_class($atts['size']);
    $gallery_div = sprintf(
        '<div id="%s" class="gallery gallery--columns-%s gallery--size-%s">',
        $selector,
        $columns,
        $size_class
    );

    /**
     * Filters the default gallery shortcode CSS styles.
     *
     * @param string Default CSS styles and opening HTML div container
     *               for the gallery shortcode output.
     */
    $output = apply_filters('gallery_style', $gallery_div);
    $i = 0;
    foreach ($attachments as $id => $attachment) {
        // Setup the image attributtes
        $image_attributes = array(
            'lightbox' => '',
        );

        // Detect caption
        $is_caption = trim($attachment->post_excerpt) ? true : false;

        // Setup image ARIA if caption presents
        if ($is_caption) {
            $caption_id = $selector . '-' . $id;
            $image_attributes['aria-describedby'] = $caption_id;
        }

        $output .= '<figure class="gallery__figure figure">';

        // Get image tag
        $image_output = wp_get_attachment_image($id, $atts['size'], false, $image_attributes);

        // Setup link
        $image_url = '';

        if (!empty($atts['link']) && !vralle_by_is_amp()) {
            if ('post' === $atts['link']) {
                $image_url = get_attachment_link($id);
            } elseif ('none' !== $atts['link']) {
                $image_url = wp_get_attachment_image_url($id, $atts['link']);
            }
        }

        // Place image tag to link, if link presents
        if (!!$image_url) {
            $image_output = sprintf(
                '<a class="figure__link" href="%s">%s</a>',
                $image_url,
                $image_output
            );
        }

        $output .= $image_output;

        // Setup caption
        if ($is_caption) {
            $output .= sprintf(
                '<figcaption id="%s" class="figure__caption">%s</figcaption>',
                $caption_id,
                wptexturize($attachment->post_excerpt)
            );
        }

        $output .= '</figure>';
    }

    $output .= '</div>';

    return $output;
}

Great work! Thanks for sharing.