Green’s Dictionary of Slang

gallon n.

SE in slang uses

In phrases

get one’s gallon (v.) [SE gallon, a container used by manual labourers for holding drinks]

(Irish) to be dismissed from a job.

1993

M. Verdon Shawlies, Echo Boys, the Marsh and the Lanes 35: After getting his gallon from Dunlop’s, he picked up a great number.