Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
We don't have any stats on how common this name is. This is probably because it's very rare in the UK.
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The Gernon surname in historical dictionaries
Patronymica Britannica (1860)
GERNON. Robert dc Gernon came into England with the Conqueror, and his descendant, Ralph de Gernon, temp. Hen. II., had two sons: 1.— Ralph, ancestor of the Gernons and Cavendishes of England ; and 2. — Roger, who accompanied Strongl>ow into Ireland, and became progenitor of the Irish Gernons still subsisting at Athcarne Castle, co. Meath. Of the locality of Gernon, whence at the Conquest the family came, I am ignorant ; but it appears not to be in Normandy. Gernun, Gernoun. H.R.