High GI carbs are good for bulking because they cause more insulin release than Low GI carbs. The insulin is anabolic and if you eat high GI carbs with a decent amount of fat (including a decent amount of sat. fat) you should be in an anabolic environment. You can still diet eating low GI carbs if you were to eat many calories non-carb food choices, but you are likely to get inferior results this way.
Most people advocate either bulking or cutting during a given time period, but not both, because it would be very difficult to achieve both muscle and fat loss unless under certain circumstances(e.g your a beginner, drugs). So, if your going to bulk, you might as well do it with all your will and if you don't eat a decent amount of non-low GI carbs then you may not achieve maximum results.