Change template if tag selected

I think the function you are looking for is has_tag() or even more generically has_term().

Your function would then become:

function get_custom_cat_template($single_template) {
     global $post;

       if ( has_tag( 'street' )) {
          $single_template = dirname( __FILE__ ) . '/street-gallery.php';
     }
     return $single_template;
}

add_filter( 'single_template', 'get_custom_cat_template' ) ;