Green’s Dictionary of Slang

1. (US) a gigolo; a dedicated womanizer.

1926		J.J. Finerty Criminalese.
1931		‘Und. “Lingo” Brought Up-to-Date’ L.A. Times 8 Nov. K3: GIG: A dance hall sheik.
1949		Monteleone Criminal Sl. (rev. edn).
1982		Eble Campus Sl. Fall 3: gig – short for gigolo. A male who has many girlfriends.

2. (Aus.) one who watches others working.

1953		T.A.G. Hungerford Riverslake 18: Randolph stood at the door and waited [...] He knew that, behind their bland glances, they were saying to themselves, ‘Who’s this gig?’.
1954		(con. 1940s) T.A.G. Hungerford Sowers of the Wind 103: Get up, you bloody gig.