A GTY is just that, guarantee of the OV cabin. The location and type of OV category is up to the cruise line.
Why would you think you could get a balcony you didn't pay for? As others have said, the inventory showing is not exact. They hold back cabins to fulfill the guarantee cabins, Royal Up and other reasons.
If you want a balcony, see if it makes sense to you financially, to just pay the difference or try to bid on Royal Up.