DCL offers two types of guaranteed bookings : GTY and IGT/OGT/VGT. The first one is not a discounted rate and has no restrictions. It also guarantees a minimum category - for example a 5B GTY will get you a 5B verandah stateroom or better - and is offered per category when that category is filled to a significant percentage, to allow life boat capacity management. A category can therefore switch to GTY on opening day.
A *GT rate is a discounted and non-refundable rate offered to fill the last remaining staterooms on the ship or if DCL finds a particular cruise isn‘t selling well enough. Most often, *GT rates are released after most cruisers have to pay their cruise in full, and once DCL knows who has confirmed their vacation. You only pick a type of room and not a minimum category. A VGT rate will therefore get you any type of verandah room.