Is there a gate I can purchase for the top of a stair case?

try a gate made for puppies. you can buy them used.

I put up a baby gate at the top of the stairs for my husband, he was afraid of the "void" as well. It's anchored to the wall and Then latches to the Newell post , so I can swing it open easily if I need to go to the basement. Makes husband feel safer.

Just put up a regular garden gate but as Pam says be sure it is securely attached. i know the feeling when you have to walk past that open void. Rather like walking over a bridge where you can see water rushing beneath.

Is there a door at top of steps? Usually there is one for the fire code. Maybe just keep it closed would suffice. I wish we could lock the door to basement at my folks, there is no reason for them to go down there, laundry is on main.

Yes there are many and readily available. The key is to mount them into a stud or doorframe for proper strength. Drywall won't hold the weight.