I book GTY most of the time, and it's been my experience that about 50% of the time I get exactly what I booked, ie: inside GTY, a decent inside cabin, about 45% of the time I get a 2 or 3 category upgrade, ie: booked an inside on Riviera deck, but got an inside, or ocean view on an upper deck, and the remaining 5% of the time I got what I considered a less than desirable inside cabin, ie: an inside cabin right at the bow, or last cabin in the stern that was noisy. In my opinion, it's well worth the risk to save the money, and besides that, it's fun, and allows more anticipation and excitement before the cruise.
P.S. Twice in many, many years of cruising, I've booked inside GTY, and got a BALCONY! Certainly not the norm, but it DOES happen.