I had the head gasket changed

It's not serious but you will want to have the cooling system flushed anyway. Petroleum products in the cooling system will rot the heater and radiator hoses from the inside. Also, if you have the red Dex-Cool antifreeze, you'll want to get rid of that and use anything else. We call that "Dex-Mud". GM also has a lot of trouble with corroded and leaking heater cores. That seems to cause more trouble in their trucks than in their cars. It is caused, in part, by the acids that build up in the coolant from the normal seepage of combustion gases into it. We replace the coolant every two years because the corrosion inhibitors and water pump lubricant additives wear out in about that much time. During the cooling system flush is when we normally flush out the reservoir too.