gistfile1.txt

<?php

add_action('init', 'register_gallery_endpoint');

function register_gallery_endpoint() {
    add_rewrite_endpoint( 'gallery', EP_PERMALINK  );
}

add_action('template_redirect', 'catch_gallery_endpoint');

function catch_gallery_endpoint() {
    global $wp_query, $post;
    // if this is not a request for json or a singular object then bail
    if ( isset( $wp_query->query_vars['gallery'] ) && is_singular() ) {
        $post = get_queried_object();
        locate_template('gallery.php', true, true);
        exit;
    }

}

?>