Have not heard of the name you mentioned (Great Allegheny Passage). The C&O Canal trail goes Wash DC to Cumberland Md, but it is not paved and mostly loose dirt. Not the greatest for weight or speed.
Pittsburgh to around Meyersdal Pa is rail trail, previous Western Marylan RR). This is in 2 to sections, Steel Valley (Pgh-Mckeesport), Youghiany (Mckee-some point) & Alleghany Train (some point to Meyersdale), the last 2 are the old RR bed, just different names. However, the last link from PA state line to Cumberland does not exist and is stepp roads.
Hope this helps. You may also be talking riding Route 30 through the mountains (Carlisle to Pittsburgh), although the grades are multiple and steep (about 8 to 10%).