I had a pressure gate that was supposd to be excellent for the top of the stairs. I got it brand new from a friend of mine and it did have on it that it was top of stairs safe. I was still trying to find one that was the right size. My daughter's friend knocked into it and if she hadn't been aware, and my daughter had not thought to try to grab her, she would have fallen down the stairs. I hate the idea that my 21 month old might have done that. This happened 2 weeks ago.
One thing you might want to consider--I certainly am--is a gate that has bars rather than a mesh or other similar pattern. I can't really think of the words to describe but a gate with bars would lesson the chance of your child climbing the gate. Believe me, there will be a day he climbs everything.
Pressure mounted gates are okay at the bottom of stairs since you will not actually fall down the stairs (used to stop your little on or pet from going up) but I am still on the lookout for a good gate that will fit my back stairs. We have an odd angle for a hardware mounted gate but that is what it is going to be. No chances.