1997 Cadillac Deville Head Gasket

After wasting money with getting a thermostat & getting my car flused, I was told that my car is overheating because of a head gasket, but wanted to get someone elses opionion. The car runs fine untill it reaches a hill, once it starts going up a hill the temperature will shoot up & cause the car to over heat. Nothing is getting mixed together & I do not have anything leaking however, the coolant is evaporating(at least from what I can tell) because I have to keep refilling the coolant. Does this sound like a head gasket issue & if so generally how much does it cost to get fixed?