There are adaptors for metal banisters.
We avoided the entire problem (and having to hardaware mount) by blocking off the hallway at the top of the stairs instead. In our house, when you come up the stairs, you reach a T intersection...you are staring at awall and can go left or right. Turn right, and about 6 inches down the hall, we used a pressure gate to close off that area. DD can't get to the stairs. Would that work in your house?
Mom to a spirited, red-headed, former 28-weeker 10/2009 and a more mellow monkey 12/2013.