Green’s Dictionary of Slang

1. (US) to stand and stare, esp. at a crime and not take part.

1898		Omaha Daily Bee 6 Aug. 12/6–7: Kansas City! Gee! there’s a quick town [...] Never see no folkses there a settin round doorsteps — gappin.
1904		C. Connors Bowery Life [ebook] Well, dey would all begin stretchin’ and gappin’.
1906		O.W. Hanley ‘Dialect Words From Southern Indiana’ in DN III:ii 119: gap at, v. To stare at. ‘Don’t be gappin’ at me.’.
1919		C.H. Darling Jargon Book 14: Gap – to look.
1934		J.L. Kuethe ‘Prison Parlance’ (in AS IX:1) 26: gap. To witness a crime without taking part in it.
1949		Monteleone Criminal Sl. (rev. edn).
1953		L. Durst Jives of Dr. Hepcat (1989) 5: An upstate guy wigs you with some most burnt toeology, the whole party is gapping and clapping making you most understand that the dancer is ace hi, on the main stem. And one ‘tip toe Joe’ that’s in the know.

2. (US drugs) to exhibit the yawning that is a symptom of the onset of withdrawal from narcotics.

[1886		‘Life in a New York Opium Den’ in T. Byrnes Professional Criminals of America 🌐 First, I began to gape and a pain in the back of the head started, then tears ran from the eyes, a catarrhal discharge from the nose, my teeth chattered and I trembled from head to foot, a cold sweat covering my body [...] I now realized for the first time that I was a victim to the opium habit].
1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 102/2: To gap. To begin to manifest withdrawal symptoms. The addict yawns frequently and may drool.

3. (US black) to look at, to read.

1939

Pittsburgh Courier (PA) 25 Nov. 7/1: I nixed the text [...] that I’m gapping and dug my conk-piece stached on memory.