Go to the Disney website's "Find a Cruise" page, and on the right click on "Show Special Offer Filters", then click on "Save Up to 35% on Select Sailings". From there follow the prompts until you see the current list of GT cruises. Some other websites also list these.
They appear at different times for different cruises- no way to know in advance. Usually they appear after the cruise's pay-in-full date, but sometimes they appear before that date.