gistfile1.php

<?php
/*
 * In template
 */
get_query_var($var);

/*
 * functions.php
 */
add_filter('rewrite_rules_array','wp_insertMyRewriteRules');
add_filter('query_vars','wp_insertMyRewriteQueryVars');
add_filter('init','flushRules');

// Remember to flush_rules() when adding rules
function flushRules(){
	global $wp_rewrite;
  $wp_rewrite->flush_rules();
}

// Adding a new rule
function wp_insertMyRewriteRules($rules)
{
	$newrules = array();
	$newrules['gallery/(.+)'] = 'index.php?pagename=gallery&pid=$matches[1]';
	$finalrules = $newrules + $rules;
        return $finalrules;
}

// Adding the var so that WP recognizes it
function wp_insertMyRewriteQueryVars($vars)
{
    array_push($vars, 'pid');
    return $vars;
}

//Stop wordpress from redirecting
remove_filter('template_redirect', 'redirect_canonical');