Coolant problem

Okay, so I have had a coolant problem on my car for a while now and it starts like this. I have had the thermostat housing replaced on it three times now in three months and it is just gone again after two weeks. So it is spraying coolant out when pressurized and I have been told that it is like an air lock in the system and the garage said that he has tried everything to bleed it and has failed so he has said that the head gasket could have blown. So I have ordered one to pick up tomorrow along with another thermostat housing but wanted to know if anyone knows if it is defo the head gasket? And the reason that the thermostat housing keeps going is because of too much pressure in the coolant system. And even when the car is stopped you can hear the coolant tank hissing with pressure slightly.