Here's a thread that has tracked *GT and other discounts offered for the past few years. You can see occasionally there are changes (I think they are marked in orange), though the few times I've paid attention that meant a different category was offered or removed, not a reduction in price.
*GT rates may come and go; it may change or simply disappear. It's sort of a "take your chances" thing - if you feel the rate is acceptable to your budget, I'd book because it may not be available tomorrow or even this evening. But if you are on the fence about that cruise anyway, and willing to lose the opportunity, feel free to wait and see if a better offer comes out.
Enjoy your cruise!