The first place you’ll want to install a baby gate, is at the top, and bottom of any stairway the baby may have access to. Gates installed at the top of stairways should always be done with hardware mountings, and not expandable pressure bars. This is for those rare instances when the bar can be jiggled loose and give way, allowing the baby to fall down the stairs. The pressure gates can be used at the bottom of stairs, however.
Baby gates today, generally come as mesh panels, vertical rail/bars that are built to government specifications regarding the width between them, or expandable accordion style gates that also must meet stringent requirements as to measurements of the diamond shapes that open up when the gate is stretched to fit a doorway.
Whichever style you choose, it should have a flat top edge, to prevent injury when a child tries to climb over it and falls on the edge. If it is a pressure bar gate, it must be placed so that the bar is on the side away from the child, because they often try to use it as a foothold to climb over top. While a pressure bar exerts considerable force against the surfaces it is placed between, it is not a guarantee that precocious children can’t find a way to release the bar.
In the event that you have an extra-wide space that needs to be blocked off, many models of baby gates offer an extension kit. However, if you find that the surfaces against which you had planned to mount the gate are insecure, such as a drywall without studs, you may also need a gate mounting kit to get a snug, and sturdy fit. In the case of banisters, you can find mounting kits that don’t need to be screwed into the wood, which will avoid marring the surface.