Rivnuts from Homestead, PA on 1/24/2022 1:38:41 PM:
The following are some of the more visible and active shuttle services along the GAP and C&O:
Golden Triangle Bike
Bill's Car and Shuttle Service
Wilderness Voyageurs
2 Wheel Escapes
You'll find descriptions and contact info for these at the gaptrail.org website under the "Where to Go" tab and the Sites and Amenities/Shuttles section therein.
As you likely know Amtrak provides bike roll-on service between Pittsburgh and DC and points along the way. There are also one-way car rental options in both Pittsburgh and DC as well.
Of course, the best option is simply to make it a round trip and avoid the need for a shuttle at all. :-)