We did all 4 Grand Gatherings this past June. The only one that was disappointing was the "Good Morning Gathering" which was basically just breakfast at Toni's Town Square Restaurant at MK followed by priority entrance to Mickey's Philharmagic.
The other 3, however, we liked better and were worth doing in my opinon.
The Magical Fireworks Voyage is basically snacks followed by a short boat ride from the Contemporary over to a prime viewing spot in the middle of the lake for the "Electrical Water Pageant" and nightly MK "Wishes" fireworks.
The Safaris Celebration Dinner was my favorite because you got to do a special slower-version of the Kilimanjaro Safari ride followed by a buffet dinner at the Tusker House Restaurant.
My 2nd favorite was the dinner and desert reception at Epcot. You start out with a buffet dinner and entertainment at the Odyssey Restaurant and then head over to a special reserved viewing area for desserts and viewing of "Illuminations".
We did the Deluxe Dining Plan also so all of these with the exception of the Magical Fireworks Voyage (because they don't serve a meal) was covered by the table service credits of the dining plan. This was a big bonus for us and I'm not sure we would have done the Grand Gatherings, or at least not all of them, if we had to pay out of pocket over and above the dining plan costs.