When creating a new sitemap the dialog has a static field for setting the mgnl:template property. See /modules/google-sitemap/dialogs/components/content/siteComponentTab/form/tabs/siteMap/fields/template. This field sets the template to a value of GoogleSiteMap which can't be found when requesting the sitemap using:

http://domain:port/<contextPath>/sitemaps/<sitemap name>.xml

In order to see this you need to create a sitemap, then go to the JCR browser and export the map and look at the mgnl:template property.