Have a 2000 Chevy 2500

Have a 2000 Chevy 2500 PU with 4WD. Truck over heated this morning. Some fluid noticed under truck. I waited for it to cool off then filled the radiator overflow reservoir with water. As fast as I put it in it came out. There's a major leak at the back side of the engine closest to the firewall. I am really hoping its not a head gasket. So far no water in the oil. Leak appears mid way up on the engine between the engine and firewall. Any idea what the problem is???