Vernon
A low gear is a truly wonderful thing, and a wonder to behold, and anything approaching 25 inches will get you up. Remember that it was originally built and designed as a route for heavily laden horses so consists of a series of steep bits then slightly easier bits so is not at all a continuous steep slog of Tremendous Cyclists' Legends, and anyway when you see the triangle warning of horses you have only a short bit to go then it eases remarkably rapidly. In my opinion it is vastly overblown, to hear some people talk it is as high as Everest, but with your bike and a bit of grit you will be fine, anyway you are with the FNRttC which gives everyone wings under their wheels.