Our family did a Grand Gathering last summer and it was the most disorganized and confusing time we have ever had with any of our trips. We have been to Disney 49 times in which one year...2002 to be exact we had 49family members travel to Disney together. That year none of our reservations got mixed up, wrong info put with wrong families and were able to book all our ADR's together without any problems whatsoever....Not so with Grand Gathering last summer. ugh!
A Grand Gathering is just a way for Disney to be able to block off rooms in the same general location...not even side by side. A Grand Gathering is also a way for Disney to help book meals for everyone to attend at the same time.
We were able to take part in an Illuminations Dinner and Dessert Party...the dinner itself was horrible, the so called show was horrible. The Dessert and location of the VIP standing area to view Illuminations were great though...right out front of the Odyssey Building...patio area down by the lake. The bad part of this night besides the actual dinner and so called show..."come on Disney you can definitely do better than this show!" was that they had the reservations for our family in the wrong name which really ticked my parents off since they paid for the whole crew to attend and get this...they lost my parents tickets so they couldn't even join us for the dinner, but yet they were the ones that paid for everyone's dinner in advance.
Our room locations were great, however, they weren't able to book the only 3 rooms we reserved side by side. No big deal, but I find it odd they couldn't do 3 rooms side by side, but yet in 2002 before Grand Gatherings we were able to get a whole floor at Wilderness Lodge reserved. Also, they had my parents room reservations in the wrong name as well and because they weren't on the list the guard had they weren't going to let them through until we figured out what name was on the list when we first arrived to check-in.
We were offered other things we could do...but it was extra money. We decided not to do the extra's for just Grand Gatherings because we had plenty of other things we enjoy doing without paying extra money.
So, it all boils down to this....No discounts whatsoever, they will offer events you can do as a Grand Gathering but it costs extra money some are worth the money others aren't, they state it helps them to book rooms side by side or in the same location and easier for them to make ADR's for the whole crew to dine together.
That's our families experience with Grand Gatherings. We won't be doing it again. We have had better luck in the past with us all booking our rooms and such separately and telling the CM to make a notation on the reservation the other family members names traveling together.
Good luck!