As others mentioned, the surface can be very different between the GAP and C&O Canal trails. The GAP is consistently surfaced with fine crushed limestone or other rock with very few sections with dirt or mud. The main issue that you might encounter on the GAP is loose gravel in spots where it is thicker. The only time that was a a problem for us was on the 20+ mile downhill stretch from the Continental Divide to Cumberland. In that section, the gravel was quite thick in spots along the lower side of the path, making traction somewhat sketchy at times. My bike fishtailed in one spot when I was passing some other cyclists where the gravel was thicker. My Pasela 32s generally performed excellent on the GAP. I personally wouldn't want to use a tire narrower than 32 on the GAP, but a cyclist familiar with riding on gravel might be comfortable on 28s.

The C&O Canal trail is quite variable in surface conditions, but generally speaking poorer than the GAP. In some locations, the surface is crushed gravel very similar to the GAP, but it can quickly change to conditions more like a dirt road or hiking path. The C&O trail also has a lot of low spots where water collects when it rains, and these spots can become quite muddy. We encountered very little rain on our week-long trip, but it had rained considerably the week before and many mud puddles remained on the trail. I would not want to ride the C&O with tires narrower than 32s, and some treads are desirable. My Paselas have a modest tread pattern and performed well most of the time on the C&O, but got a little slippery in the muddy spots. Nothing that I couldn't handle, but I had to be more cautious when riding through wet areas. I could have used 35 mm cross tires with treads on my touring bike, but would have had to remove my fenders to use them. I am glad that I kept the fenders because I (and my bike) would have become quite muddy riding on the C&O without them.

IMHO, the biggest hazard on the GAP and C&O are sticks that fall on the path from surrounding trees. It would be very easy to kick up a stick into your spokes or derailleur, causing serious damage. We saw a wheel in one of the nearby bike shops that had half of the spokes turn out at the rim from a stick that jammed in the wheel. At one point on the C&O, one of my riding companions told me to slow down because it looked like something was jammed in my rear wheel. When I stopped to look, there was a fairly large stick jammed in the spokes, fortunately in line with the wheel rotation so it didn't break anything. If that stock had jammed sideways, it would have destroyed my wheel, and we were very far from any towns at that particular spot.