Как сделать галерею на wordpress с magnific popup?

или Как сделать свою обертку для стандартной галереи WP?

Примерно, так:

function my_post_gallery($output, $attr) {
    $args = array(
        'include' => $attr['include'],
        'post_status' => 'inherit',
        'post_type' => 'attachment',
        'post_mime_type' => 'image'
    );
    if (isset($attr['orderby'])) {
        $args['orderby'] = $attr['orderby'];
    }
    $_attachments = get_posts($args);
    $attachments = array();
    foreach ($_attachments as $key => $val) {
        $attachments[$val->ID] = $_attachments[$key];
    }
    if (empty($attachments)) {
        return '';
    }
    ob_start();
    ?>
    <div class="post-slider popup-gallery">
        <?php foreach ($attachments as $i => $image) : ?>
            <div class="slider-item">
                <a href="<?php echo wp_get_attachment_image_src($image->ID, 'full')[0]; ?>" title="<?php echo wp_get_attachment_caption($image->ID); ?>">
                    <img src="<?php echo wp_get_attachment_image_src($image->ID, 'full')[0]; ?>" alt="">
                </a>
                <p class="slider-item-title"><?php echo wp_get_attachment_caption($image->ID); ?></p>
            </div>
        <?php endforeach; ?>
    </div>
    <?php
    return ob_get_clean();
}
add_filter('post_gallery', 'my_post_gallery', 10, 2);