Gulf Islands National Seashore is more than just the area at the west end of Santa Rosa Island where Fort Pickens and the 3 campgrounds are located. The Seashore is scattered along a good chunk of the Gulf coast.
Joel is correct in his description of the part of the Seashore and campgrounds that make-up what is commonly referred to as Fort Pickens.
Beautiful area. I live almost as far west as you can go on the Island before you hit Park land. Love it out here. If you plan to arrive on a weekend I'd say reservations are almost a "must" depending on any special events that might be taking place on the Island that weekend. Mid-week you'd probably be okay, but if you want a particular size spot, I'd play it safe and reserve ahead.
Hope you come and enjoy your stay. Let me know if you have any questions.
Fran & Tom/ 4 rescue cats - Benci (2014), Peaches (2003), Tipper (2002), Snippet (2002), & in spirit - Aja (1996-2014)/2011 Winnebago Vista 30W/2008 4 door Wrangler 4X4