I have booked/done a Grand Gatherings before. But I only know a little about the experiences. It doesn't cost any extra to book a Grand Gathering, but the Grand Gathering experiences(offerings) do cost extra. I believe there is a cruise on the lagoon, a safari and meal, a meal at Epcot?, things like that. There is a minimum # of people in your party that have to attend for you to reserve that part. When we have done the Grand Gathering before there were 13 in our group, but only 5 wanted to do one of the offerings so we didn't do it. But we did use the Grand Gatherings to book our ADRs. I know there are many DISers that have gone and they could give you a better idea.