When I did the GAP and C&O for the first time I rode Marathon Plus Tour, the super durable and heavy tires. You do not need them for these trails. The trails do not have glass shards or wires from car tires like roads do. I liked Vittoria Voyager Hypers, but they are discontinued and no longer available.
Something like
The Everwear in 42mm should work - it is on sale for $20. Schwalbe Hurricane or Marathon Supreme (discontinued as well) worked great for me too.
You probably do not need a knobby tire for the C&O, knobs can pick up and scatter mud more efficiently.
In any case, I take a spare tire in addition to spare tubes. In 2018 I rode the C&O in a downpour for the whole day on Conti Speed Rides (quite light tires) and the rear tire exploded tearing the sidewall. Fortunately, this happened right near Cumberland and I was able to walk the bike to the shop for 1/4 mile.