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

To @s_ha_dum:

function add_rel_to_gallery($link, $id) {
  return str_replace('<a href=', '<a data-fancybox="true" rel="group-'.$id.'" href=', $link);
}
add_filter( 'wp_get_attachment_link', 'add_rel_to_gallery', 10, 2 );

Output:

<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-1461" href="http://streetball.name/wp-content/uploads/2013/04/ult-collection_sh10153_1.jpeg"><img src="http://streetball.name/wp-content/uploads/2013/04/ult-collection_sh10153_1-150x150.jpeg" class="attachment-thumbnail" alt="ult-collection_sh10153_1" height="150" width="150"></a>
      </dt></dl><dl class="gallery-item">
      <dt class="gallery-icon landscape">
        <a data-fancybox="true" rel="group-1467" href="http://streetball.name/wp-content/uploads/2013/04/ult-collection_sh10348_1.jpeg"><img src="http://streetball.name/wp-content/uploads/2013/04/ult-collection_sh10348_1-150x150.jpeg" class="attachment-thumbnail" alt="ult-collection_sh10348_1" height="150" width="150"></a>
      </dt></dl>
</div>