I did the opposite in September. Took the GAP to Cumberland, U.S. 220 to Bedford and then PA Bike Route S (modified from Lancaster County east) to Philly, getting into town via the Schuykill River Trail. Another modification I made to Route S is that I rode the 8 mile stretch of abandoned PA Turnpike. It's a neat ride with two unlit tunnels that require a good light. Camping opportunities along the entire route, including three state parks, on Route S, although twice I had to go off route a few miles to the campgrounds.

You definitely need to be able to handle hills. One advantage to Route S is that it was perfectly signed, which made navigation easy. If you are planning on cooking there will be a couple instances when you have to shop and carry food with you to the campgrounds. GAP-wise I stayed at the free campground (with Adirondack shelters) in Connellsville and at Husky Haven along the trail in Rockwood.