As the PP stated, your GG# allows you access (for an additional fee) to select events (an International Dinner & Illuminations Desert Reception at Epcot for example. It helps the room assigners keep your group together. And, if you are booking TS meals, be sure to use your GG# - again, it helps to keep your group together (at one table) or at least close to each other at nearby tables.
Grand Gatherings are as much or as little as you make of them ... some groups opt to get personalized shirts, etc. (can buy through Disney or create your own). How much "stuff" your group wants to do together will determine how much work organizing your GG will be - juggling a bunch of different opinions/wants/needs can always be a challenge.