I am pretty sure they are first come first served. They are quite popular and are usually full most weekends starting on Friday night. There are a couple of options on the north end of Jefferson Notch road that are not managed but used most weekends once the road is open.
As I've mentioned elsewhere on this forum, the WMNF has never had the authority to charge a fee for undeveloped campsites such as those found on the Gale River, Haystack, & Cherry Mtn. roads. Read FLREA for yourself. The relevant passages in this instance are: Section 803(d)(1)(E) & Section 803(g)(3)(A).
The fact that the WMNF is dropping the camping fees in these areas means that they are only just coming into compliance with the law.