Now i have this snippet in my functions.php

    /**
     * Add rel to links.
     */
    function add_rel_to_gallery($link) {
        return str_replace('<a href=', '<a data-fancybox="true" rel="group" href=', $link);
    }
    add_filter( 'wp_get_attachment_link', 'add_rel_to_gallery' );

Every link in gallery has **rel="group"**, but actually i want get something like that:

    rel="group-$gallery-id"

How do this?

**What is gallery id?**

    <div id="gallery-1" class="gallery galleryid-1457 gallery-columns-5 gallery-size-thumbnail">
        <dl class="gallery-item">
          <dt class="gallery-icon landscape">
            <a data-fancybox="true" rel="group" href="smt.jpg"><img src="smt.jpg" class="attachment-thumbnail" alt="ult-collection_sh10154" height="150" width="150"></a>
          </dt></dl><dl class="gallery-item">
          <dt class="gallery-icon landscape">
            <a data-fancybox="true" rel="group" href="smt.jpg"><img src="smt.jpg" class="attachment-thumbnail" alt="ult-collection_sh10153_1" height="150" width="150"></a>
          </dt>
        </dl>
    </div>

In this output gallery id is 1457.

I need gallery id (or something that can grouped images from one gallery to one rel group) for create fancybox groups: 

> Note: ID's are single use and are only applied to one element.
Galleries are created from elements who have the same "data-fancybox-group" or "rel" attribute value. 

via http://fancyapps.com/fancybox/#examples