*GT rates are offered on those cruises that
DCL deems as not selling well. No way of knowing what cruises they may come out on.
I will point out that GTY is not the same as *GT. Once a certain percentage of rooms within a specific category have been "sold" the category goes GTY. That means you can still select that category, but you cannot select a specific room. GTY rates are the same as if you were able to select a specific room. *GT rates typically come out after the Paid In Full date and you book a TYPE of room, not a category.
I'll also point out that cruisefish is a little obscure about how many rooms are still available. When it shows 65% availability, it means that 65% of the CATEGORIES are available, not rooms. As long as at least one room within a category is available, that category is considered 100% available. It's quite likely for there to be fewer rooms available with 65% availability than there are with 20% availability.