My car is overheating and I have done all the checks. It is showing nothing. If I need to repair the head gasket
Or replace it how much will it cost?

Figure $2,500, if you need to replace the head gasket, machine the warped aluminum head, and repair any damaged valves.

Well, depends of your car but I got an holden astra and I needed to fix the head gasket too. In some garages they told me $1,500 AUD (it was in Australia) and we manage to fix it for $1,000 AUD.
I'm not an expert so I just tell you how much it cost us, and it was in Australia so maybe it's not the same price too

My vw beetle 2000 is over heated, I changed the thermostat, radiator and is still overheated. Now my mecanic told me it is the head gasket, should I believe him

Be prepared for further surprises. Head gaskets blow for a reason, which is usually because the aluminum head warps. It's also common for valves to be damaged.