Error code read P1176

Question

My car runs super well, lots of power, very smooth. Approximately once every 6 weeks the dash light for the water lights up because the coolant in the reserve has gone below the sensor. I have been adding enough water to make dash light shut off. A little less than a pint low every couple months. thought it may be slow leak where sending unit plugs into radiator. A mechanic just told me the head gasket may be blown. Could this be possible even though temperature has never gone past halfway mark on gauge? Is there a way for me to figure out if it's blown without taking it to shop?