you v got a leek just take it to a garage and say your water keeps emptying like every time you go for a drive it might be a conecntions come loose it should cost no more than $60 dollars but i would try all the connections to the pipes tightening them might do it and save you cash
to a GM dealer? may be a leaking Head gasket or Intake Gasket Take it and have a Pressure test done on your cooling system should tell where the week is
Does the coolant leak when parked or just when running.Pressure test cooling system will tell you but I don't think you are going to like what you find. Bob