When you buy a guarantee cabin, basically you're agreeing to let the cruise line assign you any room they want, providing it meets the minimum standard of the room type you've chosen.
So, if you buy an oceanview room GTY cabin, it means they have to assign you a cabin with an oceanview, but it could be any kind of oceanview cabin on the ship, anywhere on the ship.
The same with a balcony GTY. They can assign you any cabin, as long as it has a balcony.
People usually buy these for two reasons. First, they are usually less expensive. In exchange for you being flexible about your cabin, the cruise line discounts the price of the GTY cabins.
Secondly, many people book them hoping for an upgrade. Since GTY and cheaper cabins are sometimes easier to sell, the cruise line will assign people who already booked GTY to the more expensive, nicer cabins and then sell the cheaper cabin slot again to a new passenger who might only be willing to book the cruise for a low price. It's not uncommon to book a GTY oceanview cabin and find yourself in one of the nicest oceanview cabins on the ship.
It's a gamble, but if you want a good price and can live with the possibility of getting the least desirable room in your category, GTY cabins can be a great deal.