Nevermind, I solved this by hand coding the post id as gallery id like so in my loop:
if ( $attachments ) {
foreach ( $attachments as $attachment ) {
$class = "post-attachment mime-" . sanitize_title( $attachment->post_mime_type );
$thumbimg = wp_get_attachment_link( $attachment->ID, 'featured-photo', true );
$thumbimg = preg_replace("/<a/","<a rel=\"lightbox[gallery-".$post->ID."]\"",$thumbimg,1);
echo '<li class="' . $class . ' data-design-thumbnail">' . $thumbimg . '</li>';
}