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' ) ;