I rode that same route last month. There is a thread about it Millvale to Ohiopyle. People gave me very helpful advice. I used a road bike and was fine, but I could see how skinny tires could be an issue after a rain.
There are some bumpy spots climbing the last few miles to OhioPyle and they would be very bothersome on the way back down with 23s but otherwise you’d be OK. I think 28 to 32 is ideal for the GAP. The C&O is much worse (gravel is bigger and less packed down; there are more ruts) and 32 is the absolute minimum there.