My suggestion is to simply find a camp ground that is relatively close to a trail head and set a date and ask who can make it.
You will never be able to get a date that is going to make everyone happy, so just set a date and pick a place that can accomodate a group of 15-20 people.
Once you pull one off....even if it is with 5-6 people, post some follow up photos to show what happened- the good, bad, and ugly.
I think that you will find that once you have some photos posted up of the event, it seems "real" to others and can help encourage more people to come out when you post the 2nd GGG.
You can definitely do it and it should not have to be a lot of work for you.....other than picking a date and spot.
Tell people when and where to meet....make everyone be responsible for their own parking fees and applicable permits, so there less pressure on you to organize it.
It is that simple....
I can tell you that the ones that I have been to have been Great....people have made friends, found hiking partners for future trips, and had a chance to check out other people's UL gear, gotten tips on how to use gear, and suggestions on where to take trips.
You can DO IT!!!!