If you are including the file, you won't need to pass the variable via the query string.
If the variable has an assigned value, it will "pass" that value into the included file, because the included file will "become" part of the script that runs it.
for instance, if you have:
<?php
// assign value to var
$page = 2;
// include file
include_once('gbook.php');
// etc...
?>
the "gbook.php" file will be embedded into the script the runs it and the value of $page will be accessible...
Hope that helps.