Loop through pages with specific template

WordPress saves page templates that is assigned to a specific page in the db in the wp_postmeta table.

These are saved as follows:

'meta_key' => '_wp_page_template',
'meta_value' => 'NAME_ OF_TEMPLATE'

With this in mind, you can loop through pages which shares a specific page template, using get_pages (You can also make use of WP_Query)

Example:

$pages = get_pages(array(
    'meta_key' => '_wp_page_template',
    'meta_value' => 'page-special.php'
));
foreach($pages as $page){
    echo $page->ID.'<br />';
    echo apply_filters( 'the_content', $page->post_content );
}