GTY means "Guarantee." In short, when you book a guarantee with a certain category of cabin, it means that you are guaranteed to be assigned at least that category, with a possibility of upgrading (albeit a slim one). They don't assign you a specific cabin right away.
Cruise lines often overbook their more popular categories, as there are always people who cancel or decide they want to pay for a better category. When cabins in these categories open up, they'll assign a room to the people with GTY status. Sometimes, you'll end up getting a higher cabin category, although I wouldn't book a guarantee counting on this. Make sure you'll be happy with the category you've booked, that way an upgrade will be a nice surprise, but the lack of one will not be a disappointment.
