I got a cheap baby gate like below:
Amazon
I used plastic ties to secure it to one of columns on one end and installed a small hook on the other to "lock" it in place. (We had stairs that didn't have any walls on either end)
Now that we moved, I'm using the one below. More expensive but works better b/c I can modify the shape/size to my needs if necessary. I got the 76" but you can add/take away a panel.
Amazon
Zoey's about 5.5 lbs and we haven't had any issues with her getting stuck.
Okay.. I measured FOR YOU!

The space between the bars are about 2.625". I measured Zoey's head it was bit less than 3" but definitely not smaller than 2.625.