Rivnuts from Homestead, PA on 7/15/2019 11:47:09 AM:
While the Hiker/Biker (H/B) campsites along the C&O Canal Trail are free there are other campsites along the C&O that are not free such as at Spring Gap, 15 Mile Creek, Antietam, etc. While they may appear to be H/B sites they are generally larger and better maintained and hence why they require a reservation and fee.
There are free trailside campsites along the GAP Trail although they are not referred to as "H/B" sites. There are free camp sites at Connellsvile, Round Bottom, Cedar Creek Park and Dravo Cemetary.