This map (PDF) shows the Clairton Connector Ahlir mentioned, to get from the Montour Trail to the GAP. It's pretty well signed, though.
As to the Montour itself, the on-road sections are pretty safe and easy. The longer ones mostly have map boxes at both ends, but it's a good idea to print out or bookmark the directions for the bypasses, particularly for the easternmost ones. (There's some trail construction not reflected on that page, but it's all well-marked.)
The airport connector is 6 miles, and it's about 39 miles on the trail to the Clairton terminus. The Clairton Connector is about 5 miles. So 50 miles total from the Airport to McKeesport and the GAP.
The most challenging bits:
Clinton Road on the Airport Connector has a few blocks with narrow shoulders (but it's downhill in your direction, so it shouldn't be a problem).
The east end of a temporary detour onto Hahn Road near the under-construction Georgetown Road bridge includes a steep 20-foot climb up some gravel. It's a bit loose, so be careful when walking up it. But the bridge will be done in a month or two, so you may not encounter it.
Library Road has some traffic, but shoulders and parking lots let you avoid it if you want.
A short unfinished stretch of trail that starts at Stewart Road can get very muddy after rains.
Piney Fork Road has an 800 foot stretch that climbs gently. Two lanes, no shoulder, but not much traffic and it's slow.
The way past Route 51 is not obvious without directions of some kind.
If you take the sidewalk on the Clairton-Glassport Bridge, getting back on the road from the east side can be tricky. Heading down the sidewalk to the next driveway is one option. You can take the road instead, where there's a good shoulder, but it's difficult getting across the traffic and into the left lane by the end of the bridge. (Waiting for the light to change and the traffic to clear, then moving to the left lane and holding it, seems to work OK.) If you like industrial vistas, you may want to take the sidewalk just to stop and look for a while.
As to taking the 28X, it's possible, but the road climbs with a 6% grade for about half a mile, and there's no sidewalk.
Finally, one warning. Last week's Montour Trail newsletter mentioned there were two projects that could temporally close the trail at Clairton (if you go that way). Any closures should be announced on their web site, but you may want to subscribe to their email discussion list too.
