Cadillac Repair: 2001 Deville DHS Overheat/Head Gasket, head bolts, head gaskets

Question
Car has 117k miles.  Temp gage increases to high & Hot Engine indicator is illuminated.  No fluids visible, but there is an odor of coolant.  Dealership replaced thermostat & coolant and added a GM Sealer.  Car still overheats.  Could the gaskets need replacement & if so, what is the cost?  Just purchased & no warranty.

Answer
This engine is FAMOUS for leaking head gaskets due to stretched aluminum threads in the engine block.

Repair is to drill out head bolts and install heli-coils in all head bolts in the block and install larger head bolts as well as the head gaskets.