'Notting' probably comes from a man called Cnotta; the 'gate' was a toll gate.
An early form, Knottynghull, appears in the Patent Rolls of 1356. For years the name was thought to be linked with Canute; now 'Nott' is thought likely to come from the Saxon personal name Cnotta, and 'ing' from the Saxon for a group of people. The gate was a turnpike at the foot of the hill on the road to Uxbridge.