2 years ago, I tried to lay out a route that traced the PA Canal. The route is north-east from Pittsburgh along the Allegheny River, and then east. I gave up on the route because I felt the state (or at least this part of the state) was not bike friendly. There are all these narrow river valleys, so the road are narrow, there are no shoulders, and the drivers go 20 MPH over whatever the posted limit is. That said, here is a web site that will help you plan your route once you get into PA:http://www.dcnr.state.pa.us/stateparks/recreation/biking.aspx
The GAP does not actually start in Pittsburgh, but in one of the suburbs. There is a route to the airport, but they discourage you from starting in Pittsburgh. Here is the GAP website: http://www.atatrail.org/
Good luck, and I would like to hear details of your route. My profile shows my private email address.