The exit from the West side bike path to street level at the GWB is very steep. There are no stairs there, IIRC. I've only done it going to the path from the GWB and I rode down a very steep path to get there. Once you get to street level you have to cross over a major road. There is a bridge with a ramp on both sides, no stairs.
It might be easier to get off the bike path earlier and then go up 165th street, left onto Ft. Washington, left on 177th, then a right on Cabrini (2 blocks down) to the south walkway entrance of the bridge.
Carol