Rivnuts from Homestead, PA on 7/25/2020 9:06:58 AM:
As noted in the earlier response, there is a Port Authority bus that runs from the airport to downtown Pittsburgh. All Port Authority buses have two bicycle racks on the front of the bus. A face mask is required to ride the buses.
If you are not committed to starting the GAP trail at its origin at Point State Park in Pittsburgh, you can ride from the airport to McKeesport via the Montour Trail and its extension to the airport and pick up the GAP trail there in McKeesport. That is about a 50 mile ride. Doing so bypasses the first 18 miles of the GAP from downtown to McKeesport.
You can, and I've done it, ride from the airport to downtown via the Montour Trail and its extension and the Three Rivers Heritage Trail. While the latter is a "trail" it is not well marked or easily followed by those not from the area. Further, nearly all of it is on busy roads albeit you can ride on a sidewalk adjacent to portions of those roads. Frankly, I wouldn't recommend this to those not familiar with the region but, if you are adventurous and not grossly risk averse, you can do it. I can provide additional details if you are interested in this route.