Adds a custom rewrite endpoint to WordPress for displaying galleries in the place of content.

rewrite-endpoint.php

<?php
/*
Plugin Name: Gallery Rewrite (for wpse27638)
Plugin URI: http://pmg.co
Description: Builds cutom rewrites to display a gallery or not.
Version: 1
Author: Christopher Davis
Author URI: http://pmg.co/people/chris
License: creative commons/GPL2
*/

register_activation_hook( __FILE__, 'wpse27638_activation' );
function wpse27638_activation()
{
	wpse27638_add_rewrite();
	flush_rewrite_rules();	
}

register_deactivation_hook( __FILE__, 'wpse27638_deactivation' );
function wpse27638_deactivation()
{
	flush_rewrite_rules();
}

add_action( 'init', 'wpse27638_add_rewrite' );
function wpse27638_add_rewrite()
{
	add_rewrite_endpoint( 'gallery', EP_PERMALINK );	
}

add_filter( 'request', 'wpse27638_request' );
function wpse27638_request( $vars )
{
	if( isset( $vars['gallery'] ) ) $vars['gallery'] = true;
	return $vars;	
}

add_filter( 'the_content', 'wpse27638_content_filter' );
function wpse27638_content_filter( $content )
{	
	if( ! is_singular() ) return $content;
	if( get_query_var( 'gallery' ) )
	{
		return '[gallery link="file"]';	
	}
	else
	{
		return $content;		
	}
}