interesting!! i haven’t found RwGPS to be off by much over any significant distance - but it’s very choppy and not accurate for very short segments.
what does strava say? you can upload a ride to the free strava and see the grades, very similar to RwGPS.
significant roads in california are rarely graded to over 8% for any length of time, but small rural/neighborhood mountain roads may well
be. i’d be pretty surprised if you have much over 10% except in an exclusively residential neighborhood.
as an example, the ride up mt tamalpais, mt diablo, mt hamilton, etc rarely exceed 8%, and when they do it’s for 50-100 feet. of course, in SF proper i live at the intersection of two streets which exceed 25% - but they don’t build cities like that any more