You can either add the link directly to your PowerGallery/galleries/grid_lightbox/index.php page after the Gallery Title:
Look for the line: <!-- gallery title --> and change the line below it by adding your link:
<!-- gallery title -->
<span class="title"><?php echo $gallery['title']; ?> [<a href="../../../index.php">Back to Home Page</a>]</span>
(Change the 'Back to Home Page' to what you want)
Or you could create (and link to from your menu) a new page with the gallery as an include, and whatever other text, links or formatting you want.