I disagree with this statement. On
DCL, once a category goes GTY (11C GTY, 9C GTY, 7A, GTY, etc.) it does not usually revert back to ability to select a stateroom. It's either accept the GTY or choose another category. GTY doesn't pop up and go away, unless maybe the category gets sold out and then a lot of cancellations later.
GTY is not a discounted fare on DCL. It is full fare, but allowing DCL to assign a stateroom in that category or higher. Since as PPs have mentioned GTY is offered once a certain percentage of staterooms in that category are booked, the fare may even be higher than passengers who booked earlier and chose their stateroom.
*GT fares are discounted. These fares may pop-up, disappear, and return (at the same or different discount). The *GT are restricted fares, typically offered within 60 days of the cruise date, and require full payment when booking with no changes/modifications allowed.
OP -- as mentioned, "GTY" allows DCL some flexibility in marketing/selling categories and adjusting stateroom assignments due to inventory fluctuation. On the Dream/Fantasy, 5E staterooms are quite popular and get snatched up quickly so it's not surprising to find that category as GTY a full year out for spring-break time. During a lower season, it might not be GTY quite as early.
Enjoy your cruise!