The Gong Pit is used to dispose of gong to protect peasantry from disease in Stronghold 2.
In Kingmaker games and certain story-line missions, gong piles appear around the castle. These piles can be removed by placing buildings over them, but they may appear in places where it is impossible to do so. Gong Pits are maintained by Gong Farmers, who are immune to disease and will clear up any nearby gong piles around them.
Eventually, the gong pit will be filled and will stop the farmer's work, so remove it immediately once it is filled. Gong Farmers may also turn to crime, so multiple gong pits are helpful in case this happens.
If gong is not cleared over time, it will occasionally emit disease which causes workers to either fall into a dizzy state (which they will recover from), cough, or die from it. Units will also be harmed by the disease, and rats can spread it around if they run into a gong pile emitting disease. Apothecaries can remove the disease, but the disease cloud will eventually dissipate.