On the west side of the Manhattan Waterfront Greenway there are public rest rooms at the 79th Street Boat Basin and at the Fort Washington Activity center. The Fort Washington Activity center is a small, stone building built into the steps near the tennis courts just before the George Washington Bridge, not far from the 181st Greeenway entrance / exit ramp.
Ramp to 181st Street from the Greenway.
The New York City entrance to the George Washington Bridge south bike path and pedestrian walkway is on 178th Street, a few blocks west of Fort Washington Ave. To reach it from the Greenway, take the 181st Street exit of the Manhattan Waterfront Greenway (north of the bridge) and then go right on Riverside Dr. Take a left on 181st St. and then right on Fort Washington Ave. At 178th St., make a right.
The GWB south bike path and pedestrian walkway is closed from 12AM (midnight) until 6AM.
As the picture shows, the ramp at the NY side is steep; an unassisted person with a handicap is very likely to find it difficult to ascend or descend safely.
The Palisades as seen from the Greenway.
Directions to the Manhattan Waterfront Greenway from the George Washington Bridge south bike / pedestrian path, NY side:
From the bottom of the bottom of the exit ramp of the George Washington Bridge south bike / pedestrian path, go right. At Fort Washington Ave, turn left and then turn left again at 181st St. 181st ends a few blocks down a hill at Riverside Drive. Turn right onto Riverside Dr. The entrance to the Greenway is about a quarter block up on the left.
The 176th steps did not seem to go to the Greenway. I turned back and found the bike route at 181st Street. The George Washington Bridge bike path / walkway that I access from NJ is on the south side of the bridge.
Found the Greenway!