There is a parking garage in downtown Pittsburgh right next to the Amtrak station. Drive there, park, get on your bike and ride to the start of the GAP if you wish, only a few blocks away. At the end of the GAP (Cumberland, MD) or at the end of the C&O in DC, hop on the Amtrak and head back north to Pittsburgh. You'll get in late, but you can head right over to the parking garage and be on your way. Definitely make reservations for you bike as early as possible as those tickets are limited and they go fast.