on the add to site page, copy the php code that is given, example:
<?php
$pg_id = '1';
$pg_theme = 'dark';
include('/Applications/MAMP/htdocs/pg/galleries/ribbon_fullsize/index.php');
?>
Open the page you wish to display the gallery. In design view, place your cursor where the gallery should display, the go to code view and paste the php code from the add to site page.