There's a bug introduced in 2.6.6 and still present in 3.0.0 that breaks the query string for gradient.php and stops it picking up any parameters except the start color.
Somehow all the '&' characters have been escaped to be '&', but because it's just a URL they never get unescaped.
In 3.0.0 it's on line 669 of custom-styles.php.
