On my post Best Baby Gate for Stairs I reviewed and recommended the Cardinal Gate's Stairway Special Safety Gate for my stairway gate. Here are a few things I considered that may help you with your own purchase...
Single hand operation
Definitely important that the gate can be opened with one hand with ease. I often have a baby or bags to bring upstairs in one hand. I sometimes have the twins in both hands making it even more important that it is possible to open with little effort.
Latch with sufficient complexity
While it is great that the gate opens with little effort, it is equally important that the latch is complex enough so that an infant or small child cannot figure it out or lacks the dexterity to perform the magic to unlock the latch puzzle.
Lightweight but strong
Many gates require a small lifting motion in order to open it. The gate should be light enough so that it doesn't feel like lifting a concrete block. Also, if installing in to drywall, there is potential for a heavier gate to loosen from the wall over time with usage. Aluminum can be a good option and it doesn't rust which is an added bonus.
Mount at different angles
Will the gate be mounted in a door jamb? If so then skip this section. If not, then make sure the gate can be mounted at different angles. It is common to do this especially when one side will be mounted to a stairway post and the other side to a wall. Also, maybe the opening that will be gated off just doesn't have parallel walls. Some gates may have limitations to where they can be mounted.
Just a tip - when planning, take a look at baseboards and other wall features that could either get in the way or may need to be worked around.
Two common types of mounts are pressure or wall mount. Some gates offer both options for flexibility. For a top of stair gate, I prefer the screwed in to the wall stability of a wall mounted gate. Pressure mounted gates are easier to install and any damage to a wall they will cause will probably be more superficial. They can also be moved from opening to opening providing the ultimate in portability/flexibility. Wall mounted gates can have a more streamline appearance because pressure mounted gates typically need the stabilizing bar that sits on floor across the bottom of the gate.
Plastic or metal parts
This can be a bit more subjective as there are pros to having plastic such as possible less noise with metal hitting plastic or plastic hitting plastic vs. metal hitting metal. However, metal parts, especially if they are stainless steel and used for the hinge area, may last longer and take more abuse.
Configure direction of gate swing
Make sure the gate swings in the direction that is needed. If the gate swings both ways and it is a top of stair gate, make sure there is an optional stopper so that it can be configured to not swing out over the stairs.
Depending on the need, it can be good to get an expandable gate. Not only is this good for larger openings but it can probably make installation on any opening easier. Just adjust the width of the gate rather than worry about implementing manufacturer or DIY spacers.
Gate can be easily removed
This is a nice to have. For me it is ok if the mounting hardware stays on the wall as long as the gate itself can be easily removed and reinstalled. This is great if I have a party and need frequent passage or movers need to move some stuff through, etc.
This is probably a given but I do see some stuff out there that is atrocious. Just because it is a baby product doesn't mean it needs to be rainbow colored. A baby gate, especially if it will double as a pet gate, may be installed for years in a house. There is nothing wrong with having it elegantly blend or tie in to the decor.
Tall enough to prevent climb over
Check the height of the gate. Some infants are tall, others are like spider monkeys, and even others become toddlers. Some manufacturers offer extra tall gates to consider.
Adaptors for mounting on stair banisters
If the gate needs to be mounted to a stair banister, ensure that there is an adaptor out there that will fit both the banister and the gate. For example, there are both iron, wood, and other types of banisters. Some posts are round and some are square. These adaptors can be a costly addition to the total price of the setup and alter both the performance and aesthetic of the setup.
Auto-close and lock or manual close and lock
I prefer a top of stair gate to not auto-close and auto-lock. We like to explicitly close and lock the gate on stairs instead of depending on auto-close/lock functionality. I view the latter being safer for non-stair gates where if the auto lock doesn't fully engage for whatever reason, it won't cause a spill down stairs.
Top of stair gate, bottom of stair gate, non-stair gate
Consider where and what you are actually using the gate for. For example, a top of stair gate should have the ability to only swing one way, mount to the wall (my preference over pressure mount for stairs), and not auto-close/lock.
Seal of certification by the Juvenile Protection Manufacturers Association
Of course, the gate should be fully tested and have a seal of certification by the JPMA. Buy current models as today's certifications may be different from yesterday. Remember when it was "ok" to ride in those backward facing seats in the rear of station wagons!
Other stair gates to consider: