.Google is wrong. 3 Rivers Heritage trail is part of the GAP. The GAP is not one contiguous trail. Telepathically it is made up of lots of smaller trail segments that have been joined together to form the GAP. Just ask the volunteers the a section of trail is called that put their hard work and sweat into the section they maintain.. You might be surprised of the answer.
And there really is no link yet from the point to the rest of the GAP. Concrete for a new ramp has been poured on the downtown side but work remains to link it with the point along the Mon. This should be complete "later this year". Options are to take street level on the downtown side or cross the Ft. Pitt Bridge to the South Side. Fort Pitt Bridge has a narrow walkway so if you are towing a trailer it could be problematic. It is a bit of a squeeze with panniers especially if someone is coming the other way.
You will want to go into town anyway to see Bicycle Heaven.
https://www.bicycleheaven.org/