Green’s Dictionary of Slang

(UK Und.) to spy on, to inform against; thus gnarling, likely to act as an informer.

1788		Sessions Papers Dec. 8: I heard Merryman and Thrust say to the accomplice, Sherrington, you B—r, what did you narle for .. . Had Sherrington at that time told ? — Yes, he had told the whole.
1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 244: To gnarl upon a person, is the same as splitting or nosing upon him; a man guilty of this treachery is called a gnarling scoundrel, &c.