Green’s Dictionary of Slang

1. (Aus.) a lookout; an informer.

1949		G. Johnston in Sun (Sydney) 16 June 3/1: Their customers come to them - although often men are employed as outside agents or touts, and other men are given small sums to act as ‘gigs’ or ‘cockatoos,’ to warn them, by an intricate tick-tack system, of the approach of police, detectives or other ‘suspicious characters.’.
1953		Baker Aus. Speaks.

2. (Aus.) a detective, or any intrusive person; an eavesdropper.

1959		Baker Drum.
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxv 6/1: gig: Stickybeak.
1968		J. Alard He who Shoots Last 2: No disrespect Bricky, I ain’t a gig. [Ibid.] 159: gig One who cannot mind his own business.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 10: Being a syrup of fig was not his go. It was always on the cards that he could end up with a bit of swish if he got sprung being a gig.

3. (Aus. prison) a visitor, esp. one seen as a voyeur.

1984		B. Ellem Doing Time 77: The visitors, or ‘gigs’ as prisoners call them, are shown only the better conditions and usually only get to talk to the more trusted prisoners.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Gig. [...] a person who comes from outside to view prison and prisoners, or a prisoner who does not mind his own business.

4. (Aus.) a visitor, a stranger.

1976

D. Ireland Glass Canoe (1982) 23: Some gig from another part of Sydney [...] would protest at Serge putting a colour on the black.