The 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 stone slabs respawn after a while it makes Gummy Slugs renewable albeit the slow process.
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).
Webber and Wilba can eat Gummy Slugs without the Sanity Penalty. Wilba will turn into a Werepig after eating two gummy slugs.