The bonus he received in addition to his salary was pure 'gravy'.
They're trying to get on board the 'gravy train'.
a 'gravy' boat [=a container for serving gravy]
(..) fella that plays the organ. It would appear somebody objected to this and wasn't able to hold their peace. Good gravy, Marie. What'd I tell you, Pop? It's like a goddamn Nicaraguan death squad. You better shit-can that blasphemy, boy. (..)2003 Kill Bill: Vol. 1