"GTY" is what happens when a certain percentage of rooms within a specific category have been booked. You can book the category (example 4A, or 9C, etc) but DCL will assign you a room within your category or higher later. The cost for this guarantee booking is the same as if you booked a specific room.
So the benefit to doing this would be taking the gamble that you might be upgraded?
Also, I'm assuming that if we book a room that sleeps 3-4, yet have 4 people on our reservation we won't be put in a room that only sleeps 3? Not that I'm above flipping a coin to see who gets to sleep on the floor, but I'm betting my kids would be.