We had a ConfigureGate in our old apartment, to create a barrier between the living room and kitchen. It was not only pretty sturdy for what it was, but it *looks* way better than most gates.
For the PP who can't attach anything to the wall, both the ConfigureGate and the Supergate XT come as play yard kits that you can add additional panels to, so maybe you could build a "wall" around the stairway? I'm envisioning you have a situation where the stairs just come down into the middle of the room? If that's not it, you can find a wall to attach to. The ConfigureGate (and maybe also the Supergate XT) have no upper limit on length; you just have to add some curvature to it if you connect more than a certain number of panels (i.e. you can't just attach 10 panels in a straight line, because then there'd be a risk of it falling over... you have to have at least a 10-degree angle every, I think, 7 panels).