Green’s Dictionary of Slang

(UK/US Und.) a view, a look, an observation; taking notice.

1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 245: gun a view; look; observation; or taking notice; as, there is a strong gun at us, means, we are strictly observed.
1914		Jackson & Hellyer Vocab. Criminal Sl. 40: gun [...] Used both as verb and noun to express action or thing. Examples: [...] ‘He’s giving us a gun.’.
1933		Ersine Und. and Prison Sl. 42: gun, n. 1. The state of being on guard.