- Get a gate right now. Any gate. Go!
- Figure out later if another product is a more suitable replacement.
Since your child is apparently highly mobile, your worries should be in the above order. You have an immediate security concern, so it's not about the best right now. Most gates aren't that expensive that one can't afford to buy another one later - and the first could be reused in another location.
I'm not sure I follow your description of the staircase, but most gates need only very little width on the wall (or whatever surface it's secured to). They can be screwed into the surface, or placed into a plastic shape that is stuck to the wall using double-sided adhesive tape. Some can be opened using an integrated door, one can't.
We have different models in our home, and I've seen several other models in friends' homes.
They all work well.
Are you still reading this, or already heading for the store? :-)
Safe mounting of the gate:
- On the drywall side, the gate should be screwed into the drywall. That is the only way to make sure it can't be shoved aside by a child; double-sided adhesive won't do because it won't stick securely to the wall paint.
- On the metal spindle, screws will definitely work but I would hesitate with this solution because you can't easily close the screw holes when the gate is removed (on drywall, this is easy to do well). Still, they're a reliable last resort. See if you could use double-sided tape first though -- that would work provided that the gate applies outward pressure when closed, and provided that the gate has a significant mounting surface to ensure a solid hold -- two coin-sized patches are not enough!