I know all these stand for guaranteed rates, but what are the differences between the abbreviations and how do I find them?
When a certain percentage of rooms within a category are booked, the category goes GTY. You still pay the same price for a, say, GTY 9B that you would if you book a specific 9B room.
The only savings comes in if, IF, DCL puts you in a room that is a higher category than you booked, you won't pay additional money for it. Most people who book GTY get the category they booked.
There is some confusion between GTY and *GT rates. The *GT rates are the last minute restricted fares that come out about 60-90 days out on those cruises that aren't selling well.
With the *GT rates you book a type of room (IGT - Interior; OGT - Oceanview; VGT - Verandah) at a discounted rate and DCL will assign you to a room of that type. Since each type of room has 2 (or more) categories within it, these rates can be a savings. However, I will point out that the *GT rates typically are higher that the rate was on the same room on opening day. There are several restrictions with the *GT bookings, including there are no refunds.
If you don't care what room you have, and can travel fairly soon after booking, these rates are probably looking into. Not all cruises will have them. You will only find the *GT rates by keeping an eye on a specific cruise that you may be interested in. They just appear (and disappear) randomly.