Summer Infant company (sold at babies r us) makes a stair, walk through kind of gate, we had one at our former apt. IT is adjustable like from 32" to much much wider, and you can buy extensions. THe gate is about 50.00 dollars, with the extensions being about 15.00 each maybe.
It is a great gate, it has a swinging door gate on it, that automatically locks itself when you go through it. There is no way a baby can undo it, and it is up and down white metal bars, so no toeholds for little monkeys.
Walk Through Stair Gate...I think that's the name. Sorry I'm fuzzy on details, it's been a while since we've used it, we moved.
It's the only stair gate you'll ever need!