Thats the thermostat housing.
If its leaking from there then you will need the gasket set for the thermostat housing, are you sure that thermostat is working correctly?
One way to test whether its opening all the way to let collant into the engine is to feel the hose on the right in the picture, if its hot then coolant is circulating. Also another way to tell is to get in the car and turn the heat on, if it blows hot air then it opens if the air stays cold even after the car should have warmed up by now then its not circulating.