Which? Magazine have recently reviewed 30 stair gates, and cover every aspect of safety, fitting etc.
Have you thought about having handrails fitted on the landing or having a lightweight walker upstairs too?
A relative of mine had a catastrophic fall 12 weeks ago on the landing, hit her head on a radiator , resulting in a fractured scull and 3 bleeds on the brain. Minimally conscious for the first 8 weeks, now emerged, but is peg fed, suprapubic catheter fitted. Now learning to swallow, use her limbs again, awaiting a place at a neuro rehab unit. Cognitive state far from her baseline, and no way of knowing how much she will recover.