1996 Subaru legacy spewing coolant/water from overflow, is it gasket or water pump?

I am at a loss for how a leaking head gasket would cause the coolant to overflow, but it may be possible.

Happened to my Subaru, I believe it depends on where the head gasket fails. Exhaust was being pushed into my radiator. Did the overflow tank smell of exhaust or have back soot in it? My symptoms were a loss of coolant. You could visually see white residue from where the coolant had been seeping out at my hose clamps, bleeder plug and rad cap Luckily my radiator survived the encounter.