I haven't done this ride, but I've ridden parts of the route. (You probably have too, since you are posting from Limerick and the CTC uses the Bike Route S.)

For the Abandoned Turnpike tunnels, bring every light you own. One is more than a mile long, and both are unlit. They are also littered with debris and the pavement is cracking.

From Rockwood to McKeesport is gravel on the Great Allegheny Passage. It's fine packed, and some people ride skinny tires on it. Aside from the SRT and Philadelphia it's probably the flattest part of the ride. There's no good road alternate along much of the GAP.