white steam is perfectly normal on startup and for another 5-10 mins, it could be working air out of the system since the job was done. Do you have any visible leaks anywhere? I would pressure test it, and see if its leaking externally, sometimes a car will leak only when driving or its so small that you dont get any drips during the time you spend looking for them. Get a pressure tester on it and see if its leaking anywhere externally, if not take it back to whoever did the headgasket and have them fix it if they did it wrong
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