Green’s Dictionary of Slang

(US black) the slave quarters; thus the black ghetto and fig. the black world.

1946		Mezzrow & Wolfe Really the Blues 45: That number is a wonderful example of what happened to the blues when they moved out of the gallion, the work-gang and the levee and rode the rods into big towns.
1967		M. Williams Jazz Masters 149: [A] blues Bechet did several years later called Out of the Gallion. (The gallion is an urban black ghetto.).
1986		D. Barker Life in Jazz 158: [B]ack of the tracks; slum, or in the gallion—where the black folks hang out.