Try the custom AI to help you find products that Reddit loves.
I’ve had the same situation. I used this product:
Safety Innovations No Hole Stairway Baby Gate Mounting Kit - Fits Posts from 2 3/4 inches to 3 5/8 inches Wide with Square or Round Tops - No Adhesives On Or Screws in Your Posts, (1-Pack) https://www.amazon.com/dp/B004CEN62O/ref=cm_sw_r_cp_api_glt_fabc_Z1G44180KAWBXQA5HDYD?_encoding=UTF8&psc=1
We bought a kit from Amazon that's similar to this, but it has a little more security than zip ties: https://www.amazon.com/Safety-Innovations-GM5-Stairway-Mountiong/dp/B004CEN62O/ref=sr_1_2?crid=2KHFVQQD370IS&keywords=post+gate+kit&qid=1650952665&sprefix=post+gate+kit%2Caps%2C47&sr=8-2
We used one of these kits that clamps to the newel post and provides a sacrificial 1x2 for attachment, plus would give a bit of stand-off if installed with a hinged (swinging) gate. You’re probably stuck with drilling on the plaster side, but drywall/plaster is easy to patch.
Safety Innovations No Hole Stairway Baby Gate Mounting Kit - Fits Posts from 2 3/4 inches to 3 5/8 inches Wide with Square or Round Tops - No Adhesives On Or Screws in Your Posts, (1-Pack) https://www.amazon.com/dp/B004CEN62O/ref=cm_sw_r_cp_api_i_SGT71A4HQYQFRTDDN7BM?_encoding=UTF8&psc=1
I asked a similar question last month because not only do we have a banister on one side, but the top of our stairs is extra wide. Someone suggested something like this that attaches to the banister without needing to screw into the wall. I guess it would depend on how sturdy your banister is but it seems to get good reviews.
Can’t tell if this would work for you with the clearance on the bottom, but we have a similar setup and this worked well for us: https://www.amazon.com/dp/B004CEN62O
Cardinal ss30 are the best gates on the market.
You would probably want a mounting kit for bannister- I like this one
They actually make kits like that on Amazon.
This is the one I used: https://www.amazon.com/Safety-Innovations-GM5-Stairway-Mountiong/dp/B004CEN62O/ref=mp_s_a_1_4?crid=1KKZSPB7S8NB4&keywords=bannister+baby+gates+adapter&qid=1636082512&qsid=138-1053138-2533667&sprefix=bannister+baby+gates%2Caps%2C103...
I then made my own gate because I could match the stain to our banister.
Here's the finished version: http://imgur.com/a/RWEzRxC
You can dm me with question if you want the build info.
Maybe something like
For the banister side.
For the wall side, maybe something like this that hinges so the angle of the wall won't matter?
This kit- (you’ll need 2)
gm6 mounting kit
This gate : cardinal ss30
Here’s the exact setup
Edited: added “you’ll need 2”
we used this kit
with this gate https://www.amazon.com/gp/product/B01MA4IPB0/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
if there is a big difference between where the stud is, you could get a thicker piece of wood to attach to the connector kit and attach it to the back side of the bannister (facing the hallway) rather than the side (facing into the staircase, how it is shown in the picture). This would effectively extend the area you have to get a gate in and match it up to the studs on the wall easier.
Maybe something like this?
It looks like it works with any gate and is just a clampy on thingy.
This attaches to the rail so that you don't have to ruin the rail with holes.
Just installed one on ours last month. They sell them on Amazon as a railing kit for your baby gate. There are different kinds but here's what we got: