Years ago, GT (restricted) rates typically weren't released until after the penalty period for changes began. More recently, they have sometimes been released earlier -- there are VGT rates available now for European cruises departing in September. Some cruises, though, don't get VGT rates at all.
If you're looking at a cruise many months away, your best bet is to book now, and watch to see if a VGT rate is offered before your final payment is due. If it is, you can cancel and rebook. If it isn't, you can decide whether you're willing to pay the rate you booked. If a rate opens up after the penalty period begins, it may save you money to cancel and rebook even with the penalty.