A Grand Gathering can be "room only" or a package, or any combination thereof. You can even have people in your Grand Gathering at a different resort.
They state that they try to get folks in a Grand Gathering as close together as possible, but even with all the GGs we have tried, they can't even seem to get us all in the same loop.
The only other advantage of grand gatherings is that you can book special dinners or experiences for your group. See Disney's Grand Gathering website for details.