Green’s Dictionary of Slang

gallery n.

In compounds

gallery 13 (n.) [SE gallery, a floor of cells + the trad. bad luck associated with the number 13]

(US Und.) a prison cemetery.

1992

Bentley & Corbett Prison Sl. 4: Gallery 13 Most maximum security prisons housing inmates who are doing long sentences have their own cemeteries [...] Inmates who die in prison and are not claimed by relatives or friends are buried there.

In phrases