GTY does not guarantee an upgrade by any means, but it does seen to increase the chances of an upgrade. We've been upgraded when we did not have the gty status.
As to when GTY is available, it seems to vary. We currently have GTY status for a September cruise, Since that is the slow (hurricane) season, I'm not at all convinced that the category is sold out. However, what does happen sometimes is that large travel agencies can book blocks of rooms at a discount, which they then re-sell to their customers. They have a certain time frame where they control these cabins (rather than
DCL controlling them), so that might be why DCL lists a category as "sold out." GTY status does vary--it might not be available now in a certian category, then show up again in a few days or weeks. I was told that this reflects many things. If a TA calls DCL to book for a customer, they are allowed to hold a cabin for 48 hours (in order to get back with their customer, secure the deposit, etc.) During this time, the cabin is listed as "sold" even though it might open up if the client doesn't take it....If someone fails to make the final payment by 60 days, their cabin opens up again. People do cancel, thus opening cabins....