I don’t think there is a set number honestly. I’ve just booked a Feb cruise which went *GT on the Treasure and it looks to be around 20% of rooms not sold, with most of those being verandahs (but they are offering IGT and OGT too.) I did the Wonder in Sept 2023 and the ship was only 2/3 full and that never went *GT, however that was the last cruise before dry dock. This years Fantasy cruise also right before dry dock went *GT and the ship was pretty full, about 85% full.