For the problem below.

Error loading stylesheet: A network error occured loading an XSLT stylesheet:http://www.xxxx.com/includes/templat...lt/css/gss.xsl
I think it's about permission. So, I did the following steps to make it work :

Look for file "/includes/modules/pages/sitemapxml/header_php.php"

Search for : $zen_SiteMapXML->setStylesheet($tpl_dir . '/gss.xsl');

Then, change to : $zen_SiteMapXML->setStylesheet('/gss.xsl');

Then, copy gss.xsl file from /includes/templates/template_default/css/gss.xsl to /gss.xsl

Generate Sitemap again. By the way, I'm using Zencart 1.3.9c