GTY stands for guarantee. IF you book a cat 6 gty, it means that you will be in a cat 6 or better room, but you will not be allowed to choose your own cabin assignment.
Thus, they were sold out of cat 6 when you called. However, they know that some people (especially on the repositioning cruises) had held cabins that they would later give up. They also knew that some people had put in multiple ressies trying to get a cheaper rate. So.....
DCL was pretty sure that it would have some cat 6 opening up, but didn't have any where they could assign a cabin number to you. Since you wanted connecting cabins, you needed a situation where you could pick a cabin assignment at the time of your booking.
You might want to call them back to ask if any cat 6 adjoining have become available and if they will allow you to change your reservation (it would save you some money.) However, the repo cruises have filled again, so it's unlikely.
Most commonly, cat 11 and 12 are available as gty as these are the entry levels. If they are selling out of cat 12, but the ship is not full, they will continue to sell a 12 gty and then continue to book cabins on the ship. Some lucky travellers will then get upgrades. Filling a cabin at any price is better for DCL than sailing with an empty cabin....
So...a gty is basically a way that allows DCL to manage their inventory to the best of their ability. It allows them to book more passengers on the ship because those people with a gty don't have any real idea what cabin or even what category they will actually sail in. For us, it's a great way to go....we've been upgraded a couple times.
The only rule is not to book something you'd be unhappy in and hope for an upgrade. Book the lowest category that will give you the vacation you want. Then, if you get upgraded, it's a bonus!
