Headgaskets are a pain, and if its loosing a gallon or so of coolant in a 24 hour period, its really bad, no matter WHICH gasket it is.
If i had the time, we could pull the heads off and look for a clean cylinder, IE headgasket broke there.
Thats the only real way to diagnose it. oh and taking the heads off is not a quick thing. When my mechanic and i did my sisters Escort wagon it took 2 hours just to get the one head of the little 1.9 I4.
If its not leaking any, Check the oil, And stop driving it immediately. Let me call my mechanic (you know him) he likes GM. and let me see what he can do for her.