Green’s Dictionary of Slang

(US) the Great American Public.

1965		Life 12 Feb. 71: The G.A.P.—the Great American Public—has been given the [wrong] impression [HDAS].
1989		R. Atkinson Long Gray Line (1990) 57: The GAP – the Great American Public – poured into the reservation for the parade.
2003		in Ralph Murphy ‘Murphy’s Laws of Songwriting’ at Ascap.com 🌐 The majority of the G.A.P. (Great American Public) just wants to hear great records.