The GAP trail is flat enough that pretty much any bike (regardless of weight) and any gearing would work. There are a few steep points off of the GAP trail that a rider might encounter for food or lodging, but the amount of elevation is small enough that even if the bike had to be walked up a steep hill for five or ten minutes, it would only be five or ten minutes.

C&O has a number of steeper sections but they are all extremely short at the locks, even if you did not have low gears they are short enough that it would not be a problem if you had some speed up before you climb them.

I rode the GAP when it had been quite wet and I thought that parts of the trail were soft, resulting in a slow ride. So, I think wider is better. There were three of us, I had 50mm wide tires, another had 47mm wide tires and one had 35mm or 37mm. All three of us felt that we had the ideal tires for that ride, but we were all carrying full panniers and camping gear. And the OP does not plan to carry much weight.

I agree a hybrid would work fine, but I would lean towards tires of at least 35 mm just in case it has been wet.