Geuther Revolving Safety Gate 67-107 Cm

It's one of very few models you can attach to a staircase if you cannot drill holes on either side (and the only one that wouldn't have a threshold at the bottom at the same time).

It's pretty much in mint condition, used it only for about 6 months. New one costs 98 Euro, happy to sell mine for 45 Euro. Pick up in Jatkasaari.