That is a little outside of the scope of what the plugin does, but I’ve hacked some code up for you that should work. Add this at the top of the gallery.php file:
<div class="order-links">
<?php
if($_GET['nggv-order'] == 'desc') {
nggv_orderImages($images, 'desc', 'last');
}else if($_GET['nggv-order'] == 'asc') {
nggv_orderImages($images, 'asc', 'first');
}
$pageUrl = get_permalink($ID);
if(strpos($pageUrl, '?') !== false) {
$pageUrl .= '&';
}else{
$pageUrl .= '?';
}
?>
<a href="<?php echo $pageUrl ?>">Default</a>
<a href="<?php echo $pageUrl ?>nggv-order=desc">Hightest Rated</a>
<a href="<?php echo $pageUrl ?>nggv-order=asc">Lowest Rated</a>
</div>
The ‘Default’ ordering can be tweaked using custom fields. You can read up on how NGG uses those on their site here.