In the old southern USA, it meant something to do with a vagina. The reason I guess this is that Memphis Minnie sang a song called 'I'm selling my pork chops (but I'm giving my gravy away)'. She often sang songs about prostitution (Can I do it for you?, Down in the Alley, Kissing in the dark). No Idea what she meant by 'gravy', but for sure it will be related.
'I'm selling my pork chops (but I'm giving my gravy away)'