The first thing is do you see coolant leaking from anywhere. If you are losing coolant that fast you have a leak somewhere especially if you don't have white smoke coming from the exhaust. I suspect the heater core is leaking. There is a drain that leads to the outside of the vehicle on the firewall. That is where the coolant is probably flowing. I really don't think this is a head gasket issue but if you don't fix the leak it will become an issue.