Gummy Slug is a MeatsFoodItem exclusive to the Hamlet DLC. They can be found under Stone Slabs. Eating them raw or cooked will restore Hunger, but at the cost of Sanity loss. Since a limited number of Stone Slabs are created upon world generation, Gummy Slugs are non-renewable.
A Gummy Slug can be combined with honey to cook a Gummy Cake in the Crock Pot, which will also reduce the Sanity of the player but will restore much more Hunger.