The GAP is a nice ride. It's popular and well supported by the communities is passes through. Plenty of places to find food and lodging/camping. Surface is primarily crushed limestone. It's mostly on former railroad rights of way, so the grading is easy. Mileage is about 150 to Cumberland. Very gradual net elevation to the eastern continental divide at around mile 125 and then an easy 25 miles down hill to Cumberland. I did it in under three full days of riding with a full load on 32c tires with little tread.
While I have never ridden it, it's my understanding that the C&O is more rustic. Most of it is dirt, and it can be very muddy after a period of heavy rain.
There is a good amount of information, including maps and services lists, for both trails here:
There is also an official National Park Service web site for the C&O Tow Path:
Pennsylvania Bike Route S uses part of the GAP:
If you don't want to ride into Pittsburgh, you could ride from Columbus to West Alexander, PA, pick up PA Route S, which joins the GAP in West Newton, PA and then stay on the GAP when Route S leaves it in Rockwood, PA. Google Maps bike directions gives a route from Columbus to West Alexander that's 178 miles with 3,888' of climbing, which is not bad.