We built our deck with a 4' wide set of stairs...anything less and it's very hard to carry up large grills, tables, etc...
But, finding a pet gate that would work on such a wide stair opening was difficult. We found this Cardinal Pet Gate with the optional Extension a perfect choice.
This gate system is very strong, coated to prevent rust from the elements, and the hinge system was easy to install. Plus the fact that this optional extension was available allowed us to have our staircase closed off. The Cardinal system has an adjustable main gate, which allows coverage over a nice range of openings. Our 4' opening was just a bit too big, thus we also purchased the extension, which bolts to the main gate making a nice solid gate. Make sure to purchase the stainless-steel screws so you can have rust free protection.
One note, the locking mechanism seemed a bit weird when I first saw it, but once installed, it is great! We also use this as a baby gate and the locking mechanism can only be opened by pushing two outer taps at the same time that you lift the gate slightly...very difficult for a toddler...impossible for our cats!