the mechanic checked the compression a few months ago
Compression very rarely tells if a head gasket is blown and should not be as a way to test. You will either need to have a cooling system pressure test done or a combustion test which checks for exhaust gas in coolant. I would recommend having both tests done. The compression test your mechanic did just checks to see if pistons, valves, and timing are good. Let me know if you have more questions, thanks Diana.
what is the price of theses tests? and Thank You!