Rename page URL

I have tried some add_rewrite_rule magic and query variables and it worked. Thanks for the help everyone.

If in case anyone want to refer the answer:

add_filter( 'query_vars', 'wpse26388_query_vars' );
function wpse26388_query_vars( $query_vars ){
    $query_vars[] = 'custom_gallery_id';
    return $query_vars;
}

add_rewrite_rule(
  'topic/([^/]+)/([^/]+)/gallery/([0-9]+)/?$',
  'index.php?pagename=gallery&custom_gallery_id=$matches[3]',
  'top' );

I was able to solve my problem with this.