Green’s Dictionary of Slang

(US Black) a share of money, a loan, a bargain.

1994

N. Heard African American Review 400: A gapper is a get-over, a way to get over, you know [...] if the two guys did a robbery [...] and the other one took the money somewhere else. You’d meet him later and say, ‘Hey, man, where’s my gapper.’ But it could also have been a loan or a bargain.