Uh oh! That had to make your heart race the first time he started doing it! And I don't see how you can stop it, since you already have a baby gate up. Good luck!
I know right! After my initial freak out I grabbed my phone real quick so I could get a picture. Thankfully my stairs only go up 4 steps until it's covered by a wall so he can't go too high. I've been trying to figure out how to cut a box and duck tape it to cover the bottom he's using to climb on. Won't that be attractive to look at! knowing him, he'll still figure out how to climb it.