"Poison Gravy" is an objective in the quest The Hunger Pangs in The Horrible Hunger of the Ravenous Wattle Gobbler DLC
Head back on through the Exploding Loadin Docks to the Cocina de los Diablos. You need to go to the left of the first ladder. Just keep going over to the left to find the Marination Vat. Go over to it and interact with it to put the 3 Poison Glands into the marinade.