Yes there are. The booking system moves a category to GTY automatically when that category is almost sold out. They remove the possibility of booking those room directly, and they therefore do not show up in inventory.
As you noticed, contrary to *GT rates, GTY is not a discounted rate, so it will cost more than less desirable rooms, especially since the pricing algorithm hikes the prices for the popular categories.
The purpose of moving a category to GTY is to make sure they respect lifeboat capacity. Other cruise lines do not have this problem since most rooms only fit 2 guests, but since most rooms on
DCL fit 3-5 guests, lifeboat capacity is more of an issue. GTY bookings allow them to bump up a family that would make them go over the capacity instead of moving a party that booked a specific room.