Green’s Dictionary of Slang

1. an eye.

1833

‘The Trotting Horse’ in Convivialist in Spedding & Watt (eds) Bawdy Songbooks (2011) IV 17: And with their gapers open, they knap the lfying dust.

2. (US gambling) a small mirror or similar used for cheating in card games.

1935		A.J. Pollock Und. Speaks.
1950		Goldin et al. DAUL 77/1: Gaper. A tiny reflector in the palm of the hand or on a ring, used in card cheating. A kibitzer wears the device, moves behind the victim of the swindle, and enables his aide in the game to view the victim’s cards clearly.

3. (US black) a mirror, a looking-glass.

1944

‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.