index.php

<?php
require_once ('../smarty/libs/Smarty.class.php');
require_once ('../cls/GuestBook.class.php');
require_once ('../conn.php');
$smarty = new Smarty;
$smarty->template_dir = '/web/templates/';
$smarty->compile_dir = '/web/templates_c/';
$smarty->config_dir = '/web/configs/';
$smarty->cache_dir = '/web/cache/';

$query = $conn->query("SELECT * FROM `gb`");  // не надо использовать звездочку
$row = $query->fetchAll(PDO::FETCH_ASSOC);                // получите сразу весь массив

$smarty->assign('row', $row);
$smarty->assign('title', "Main page");
$smarty->display('templates/index.tpl');