Is it a good idea to use a sealant product for a blown head gasket?

What is your question?

My truck loses coolant and overheats. I just bought it and it is still under warrenty. The dealers mechanic says the head gasket is fine but that I need to replace the thermostat and fix a leak in the radiator. Will a product like Blue Devil head gasket sealant fix my problem as I really cant afford the head gasket repair costs.