Baby gate (Mississaga St. W., Orillia)

I need a baby gate, for a cat to prevent him from going into rooms I don't want him in. Has to be one that isn't something he couldn't climb or squeeze between bars or hop on top of it. Size matters, needs to be expandable to fit 40x40 inches or 102x102cm. I don't know if there is such a thing this size, but I can improvise with your suggestions. It will be going into an opening at the top of stairs, one side drywall, the other side stairway railing.
I will pick up anywhere in Orillia. Thank you!