If a restricted fare is available on a cruise, you will see it listed as an Inside/Oceanview/Verandah With Restrictions after you click on the room type you are looking for on that particular sailing.
You could also use CruseFish.net and check each sail date for whether or not *GT rates are offered, though sometimes it isn't right up to date.
I don't know of anywhere that just lists all available *GT rates at a given time, best bet is to watch the sailings on the DCL site to see if they pop up. They are usually only available if a particular cruise isn't selling well, and usually after the PIF date for the cruise.
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