My short answer is: whatever tire you like on the road will be fine on that section of the GAP if there hasn't been heavy rain the week before.
But I found it more fun on wider 32mm. I did most of that stretch on 28mm Continental inverted tread tires and hit a few muddy short muddy spots where I wished I'd put the bigger tires. The were also a number of deeper sand/gravel areas at various crossing points. All in all, a pretty small percentage of the mileage.
A few years later I did the West Newton to Connelsville stretch on a different bike with 32 mm tires but under similar conditions and I liked the ride overall better on those tires on both the normal good GAP surface and the few dodgier sections. Used 32mm every time on the GAP since but I've also moved up to 32mm on my road bike, too - better ride, fewer flats, same if not reduced rolling resistance.