Only ever seen a zs with the v6 do head gasket after a thermostat housing failed and didnt get noticed.
Zts and 75s with the v6 do head gaskets all the time, and i believe this is now down to the air circulation within the engine bay, and the split heater matrixes, however, i would say that the initial start of hgf is down to a water leak of some description.
Like said, try a new cap and look at the thermostat housing in the vee.
If it has caused hgf as you have found it too late, then in my opinion it would be cheaper to get another engine or car.
George
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