D.C.
When my oldest was little we also used a superyard which can be closed into a playpen type of enclosure or stretched out as a very long gate. We blocked off the fireplace and the computer desk with ours and it worked very well. All you'd need to do is tie wrap it to the railing and attach it to the wall somehow and it would work fine as the sections came apart rather easily to use as a gate. Either that or you need to find some way to pad the floor at the bottom of the stairs.