http://doc.silverstripe.org/doku.php?id=modules:googlesitemaps

"In the CMS, in the “Metadata” tab, you can click on ‘Advanced’ and set the page importance manually, including requesting to have the page excluded from the google sitemap."

Using SilverStripe 2.3.1, there is no 'Advanced' tab under "Metadata"... I'm trying to set a couple pages to 0 priority to remove them from the sitemap.xml but I can't because there is no tab ._.;;

I have the exact same issue here. Please help!

It works with the following code, apart from one thing. When changing 'Page priority' it will be saved to the database, but the next time you look in the CMS the default 'Page priority' will be shown.

code/GoogleSitemap.php

<?php
/**
 * Initial implementation of Sitemap support.
 * GoogleSitemap should handle requests to 'sitemap.xml'
 * the other two classes are used to render the sitemap.
 * 
 * You can notify ("ping") Google about a changed sitemap
 * automatically whenever a new page is published or unpublished.
 * By default, Google is not notified, and will pick up your new
 * sitemap whenever the GoogleBot visits your website.
 * 
 * Enabling notification of Google after every publish (in your _config.php):
 * <example
 * GoogleSitemap::enable_google_notificaton();
 * </example>
 * 
 * @see http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=34609
 * 
 * @package sapphire
 * @subpackage misc
 */
class GoogleSitemap extends Controller {
	
	/**
	 * @var boolean
	 */
	protected static $enabled = true;
	
	/**
	 * @var DataObjectSet
	 */
	protected $Pages;
	
	/**
	 * @var boolean
	 */
	protected static $google_notification_enabled = false;
	
	public function Items() {
		$this->Pages = Versioned::get_by_stage('SiteTree', 'Live');

		$newPages = new DataObjectSet();
		
		foreach($this->Pages as $page) {
			// Only include pages from this host and pages which are not an instance of ErrorPage 
			if(parse_url($page->AbsoluteLink(), PHP_URL_HOST) == $_SERVER['HTTP_HOST'] && !($page instanceof ErrorPage)) {

				// If the page has been set to 0 priority, we set a flag so it won't be included
				if($page->canView() && (!isset($page->Priority) || $page->Priority > 0)) { 
					// The one field that isn't easy to deal with in the template is
					// Change frequency, so we set that here.
					$properties = $page->toMap();
					$created = new SSDatetime();
					$created->value = $properties['Created'];
					$now = new SSDatetime();
					$now->value = date('Y-m-d H:i:s');
					$versions = $properties['Version'];
					$timediff = $now->format('U') - $created->format('U');
			
					// Check how many revisions have been made over the lifetime of the
					// Page for a rough estimate of it's changing frequency.
			
					$period = $timediff / ($versions + 1);
			
					if($period > 60*60*24*365) { // > 1 year
						$page->ChangeFreq='yearly';
					} elseif($period > 60*60*24*30) { // > ~1 month
						$page->ChangeFreq='monthly';
					} elseif($period > 60*60*24*7) { // > 1 week
						$page->ChangeFreq='weekly';
					} elseif($period > 60*60*24) { // > 1 day
						$page->ChangeFreq='daily';
					} elseif($period > 60*60) { // > 1 hour
						$page->ChangeFreq='hourly';
					} else { // < 1 hour
						$page->ChangeFreq='always';
					}
				
					$newPages->push($page);
				}
			}
		}
		return $newPages;
	}
	
	/**
	 * Notifies Google about changes to your sitemap.
	 * Triggered automatically on every publish/unpublish of a page.
	 * This behaviour is disabled by default, enable with:
	 * GoogleSitemap::enable_google_notificaton();
	 * 
	 * If the site is in "dev-mode", no ping will be sent regardless wether
	 * the Google notification is enabled.
	 * 
	 * @return string Response text
	 */
	static function ping() {
		if(!self::$enabled) return false;
		
		//Don't ping if the site has disabled it, or if the site is in dev mode
		if(!GoogleSitemap::$google_notification_enabled || Director::isDev())
			return;
			
		$location = urlencode(Director::absoluteBaseURL() . '/sitemap.xml');
		
		$response = HTTP::sendRequest("www.google.com", "/webmasters/sitemaps/ping",
			"sitemap=" . $location);
			
		return $response;
	}
	
	/**
	 * Enable pings to google.com whenever sitemap changes.
	 */
	public static function enable_google_notification() {
		self::$google_notification_enabled = true;
	}
	
	/**
	 * Disables pings to google when the sitemap changes.
	 */
	public static function disable_google_notification() {
		self::$google_notification_enabled = false;
	}
	
	function index($url) {
		if(self::$enabled) {
			SSViewer::set_source_file_comments(false);
			// We need to override the default content-type
			ContentNegotiator::disable();
			header('Content-type: application/xml; charset="utf-8"');

			// But we want to still render.
			return array();
		} else {
			return new HTTPResponse('Not allowed', 405);
		}

	}
	
	public static function enable() {
		self::$enabled = true;
	}
	
	public static function disable() {
		self::$enabled = false;
	}
}

?>

code/GoogleSitemapDecorator.php

<?php

/**
 * @package sapphire
 * @subpackage misc
 */
class GoogleSitemapDecorator extends SiteTreeDecorator {
	
	function extraStatics() {
		return array(
			'db' => array(
				"Priority" => "Float",
			)
		);
	}
	
	function updateCMSFields(&$fields) {
		$pagePriorities = array(
			'' => _t('SiteTree.PRIORITYAUTOSET','Auto-set based on page depth'),
			'-1' => _t('SiteTree.PRIORITYNOTINDEXED', "Not indexed"), // We set this to -ve one because a blank value implies auto-generation of Priority
			'1.0' => '1 - ' . _t('SiteTree.PRIORITYMOSTIMPORTANT', "Most important"),
			'0.9' => '2',
			'0.8' => '3',
			'0.7' => '4',
			'0.6' => '5',
			'0.5' => '6',
			'0.4' => '7',
			'0.3' => '8',
			'0.2' => '9',
			'0.1' => '10 - ' . _t('SiteTree.PRIORITYLEASTIMPORTANT', "Least important")
		);
		
		$tabset = $fields->findOrMakeTab('Root.Content');
		$tabset->push(
			$addTab = new Tab(
				'GoogleSitemap',
				_t('SiteTree.TABGOOGLESITEMAP', 'Google Sitemap'),
				new LiteralField(
					"GoogleSitemapIntro", 
					"<p>" .
					sprintf(
						_t(
							'SiteTree.METANOTEPRIORITY', 
							"Manually specify a Google Sitemaps priority for this page (%s)"
						),
						'<a href="https://www.google.com/webmasters/tools/docs/en/protocol.html#prioritydef">?</a>'
					) .
					"</p>"
				), 
				new DropdownField("Priority", $this->owner->fieldLabel('Priority'), $pagePriorities)
			)
		);
	}
	
	function updateFieldLabels(&$labels) {
		parent::updateFieldLabels($labels);
		
		$labels['Priority'] = _t('SiteTree.METAPAGEPRIO', "Page Priority");
	}
	
	function onAfterPublish() {
		GoogleSitemap::ping();
	}
	
	function onAfterUnpublish() {
		GoogleSitemap::ping();
	}
	
	/**
	 * The default value of the priority field depends on the depth of the page in
	 * the site tree, so it must be calculated dynamically.
	 */
	function getPriority() {		
		if(!$this->owner->getField('Priority')) {
			$parentStack = $this->owner->parentStack();
			$numParents = is_array($parentStack) ? count($parentStack) - 1: 0;
			return max(0.1, 1.0 - ($numParents / 10));
		} elseif($this->owner->getField('Priority') == -1) {
			return 0;
		} else {
			return $this->owner->getField('Priority');
		}
	}
}

?>

_config.php

<?php
Director::addRules(10, array(
	'sitemap.xml' => 'GoogleSitemap',
));
Object::add_extension('SiteTree', 'GoogleSitemapDecorator');
?>

I had exactly the same issue with silverstripe 2.4

and finally I found a solution that worked:
thanks to: https://github.com/silverstripe-labs/silverstripe-googlesitemaps/issues/8

In GoogleSitemapDecorator.php:

function getPriority() {

if(!$this->owner->getField('Priority')) {

$parentStack = $this->owner->parentStack(); $numParents = is_array($parentStack) ? count($parentStack) - 1: 0; return max(0.1, 1.0 - ($numParents / 10));

} else {

return $this->owner->getField('Priority');

}

}

The only additional change I made was:

$pagePriorities = array(
I deleted the '-1' => _t('SiteTree.PRIORITYAUTOSET','Auto-set based on page depth'), and set this as default:
'-1' => _t('SiteTree.PRIORITYNOTINDEXED', "Not indexed"), // We set this to -ve one because a blank value implies auto-generation of Priority
'1.0' => '1 - ' . _t('SiteTree.PRIORITYMOSTIMPORTANT', "Most important"),
.....

I hope that will help some other as well.