Why is car coolant pouring out of the overflow tube?

The best way to determine if head gaskets are involved is to give your engine a compression test. It's a fairly cheap an easy test, you can even buy the apparatus and test it yourself, just have to remove spark plugs. If any cylinder measures 10% less than other cylinders, it's likely the head gasket has blown. A blown head gasket can over-pressurize a cooling system, forcing coolant out as you're seeing.

my Cadillac is overheating and is coming out of overflow jug could you tell me what's wrong with it I put the thermostat I got the radiator change is anything else you can tell me