GTY booking become available when nearly all rooms in a category have been assigned, and *GT bookings are offered when a ship or a category is not filling as quickly as
Disney Cruise Line would like.
DCL wants to fill as many cabins as possible on each sailing. If the sailing fills up, there will be just a few rooms available, and it will be easy to assign GTY (and *GT) bookings to the available rooms. If the ship isn't filling up and DCL thinks there is a greater chance of selling rooms in lower categories, some people booked in those categories may receive an upgrade or be offered an upgrade at a reduced cost. This makes it more difficult to know which rooms are still available for GTY/*GT bookings.
DCL knows how many rooms have been sold and in which categories, so (barring rare major problems that make rooms unusable) they will not sell more rooms at or above a category than they have booking for in that category. Unlike airlines and hotels, they don't count on guests cancelling right before sailing. Because the last date to book a cabin is 3 days before sailing, they are usually able to know all available cabins and assign GTY and *GT rooms at that time. However, they do not promise room assignments until sailing, and there's not much you can adjust in planning based on your cabin location anyhow.
You will have a veranda cabin, even if it's a surprise which one.