1. Create an RSS template (available in default templates since update 47) with content like:

<!--default_1-10-->

and a sub-template according to RSS format. For example:

<item>
<title><!--ALT--></title>
<description><!--DESCRIPTION--></description>
<link>http://<?=$_SERVER['HTTP_HOST']?>/gallery/<!--SAFE_DESC-->/<!--GALLERY_ID-->/index.html</link>
<content:encoded><![CDATA[<a href="http://<?=$_SERVER['HTTP_HOST']?>/gallery/<!--SAFE_DESC-->/<!--GALLERY_ID-->/index.html" target=_blank title=""><img src="<!--THUMB_URL-->" border=0 alt=""></a>]]></content:encoded>
<guid isPermaLink="true">http://<?=$_SERVER['HTTP_HOST']?>/gallery/<!--SAFE_DESC-->/<!--GALLERY_ID-->/index.html</guid>
<pubDate><?=date('r', strtotime('<!--ACTIVATION_DATE-->'))?></pubDate>
</item>

2. Create an rss.php file at the domain root:

<?php

define('CACHE_TIME', 86400);
$_GET['force_template'] = 'rss';
$_GET['skip_cell_settings'] = true; // skips cell K calculation - saves a bit of processor time
// $_GET['custom_galleries'] = true; // uncomment this line if you need only custom galleries in site map
include('./scj/tube/index.php');