Rivnuts from Homestead, PA on 3/8/2021 7:29:19 PM:
Just to follow John W’s remarks and specific to your question, you can ride the GAP in 28mm road or hybrid tires in all but the worst of weather, albeit wider is always generally more comfortable. As John notes, the C&O Towpath is a mixed bag of surfaces and conditions and vulnerability to rain. If you plan to ride the entire length, in all but the driest of weather, I’d recommend a minimum 32mm tires and maybe even 35’s. Knobby tires would be good except, perhaps, on the paved WMRT if you choose to ride it, where road tires would be sufficient. In the worst sections under the worst of conditions, a fat bike would be much appreciated. :-)
Fortunately, recent resurfacing of the C&O between Shepherdstown and Edward’s Ferry has improved a significant portion of the C&O.