You'll usually know If the head gasket is gone as it'll probably do one or some of the following.
Blow coolant out of your reservoir, if you don't physically see it you'll notice a drop in reserve.
White smoke on start up if it's leaking back into the cylinder.
Dirty coolant, usually a light brown from carbon deposits as the burnt gas is pushed into the cooling system.
My money is on the thermostat being wide open and you had it running too long with the cap off. Unless it's a rather large leak I don't think it would gusher out like you mentioned, but tht water pump will have it gushing.
Good luck with it!
Has just realized that I don't care what it costs until I get the bill! :roll eyes: ...