I did the GAP and C&O last year and it was a great trip. I highly recommend for all cyclists. I did it over 6 days and used trip planner, Noble Invention. They do a lot of trips on the trail and know all of the options. See link below:
https://www.nobleinventionbiketouring.com/
We stayed in B and Bs and some hotels. Noble will customize the trip for you and they have shuttles back to Pittsburgh from DC if needed. They can do camping trips as well. I would call Noble to discuss your plan. They have a lot of youtube videos on the trail as well.
Happy to discuss by phone if that helps.
A good step is to get the trail guide which is quite comprehensive.
https://gaptrail.org/before-you-go/trailguide/
I rode a gravel bike w 32mm tires and did ok. People were on everything from mountain bikes to road bikes but a lot of this is weather dependent. GAP is a very well maintained trail but C&O can get rougher. Miles are much slower than on the road.
It was a very fun week and well worth the effort. PM me for more details.
Alan