Did the temperature warning light come on before you had the head gasket renewed? Did the engine overheat and cause the head gasket to fail? Or did the warning light appear on after the garage repaired the head gasket? If its the latter, then you should continue to revisit the garage to get them to rectify.

We do not have wiring diagrams for specific cars, however I would assume that if they have changed the sensor, which works from the temperature of the engine, then it may be an electrical fault. Does the red light come on when you switch the ignition on and stay on all the time, or does it come on when the engine is hot? I would suggest that a temperature check is carried out as soon as possible by the garage, to establish if the car is actually overheating or not.