Gummy Slug is a FoodItem exclusive to the Hamlet DLC, which can be found exclusively under Stone Slabs. Eating one, raw or cooked, will restore Hunger, but at the cost of Sanity. Since a limited number of Stone Slabs are created upon world generation, Gummy Slugs are non-renewable.
A Gummy Slug can be combined with a sweetener to cook a Gummy Cake in the Crock Pot. The Gummy Cake will also reduce the Sanity of the player but will restore much more Hunger (the same amount as a Meaty Stew).