GTY status of a specific room category, meaning the specific room isn’t assigned at booking but the cost is the same or just slightly lower than for previously booked rooms in that category, indicates that most cabins in that category have been sold and
DCL wants to be able to move people around to the remaining rooms for balance or operational needs. Someone booking GTY will get the category booked or “better” (meaning a lower category number). For example, someone booking Category 7 Guarantee could be placed in a Category 6 anywhere on the ship if one of the Category 7 rooms can’t be used, but will probably be placed in a Cat 7.
In the past, once a certain number of rooms in a category were filled and the category moved to GTY, it was no longer possible to book a specific room in that category. I do not know if this is still true.