Green’s Dictionary of Slang

a ‘knowing one’, thus a cheat or sharper; thus gnostically adv., knowingly, artfully.

1819		‘One of the Fancy’ Tom Crib’s Memorial to Congress xxvii: Many of the words used by the Canting Beggars in Beaumont and Fletcher, [...] are still to be heard among the Gnostics of Dyot-street. [Ibid.] 14: Which, of course, made the Gnostics on t’other side flinch.
1821		Flash Dict.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
1859, 1860		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1874		Cairo Bull. (Cairo, IL) 5 Nov. 2/3: [from The Graphic, London] Farewell, gonnoffs and gnostics all, / And gillies sweet and free.
1881		Trumble Sl. Dict. (1890).