Why is my 05 Pontiac G6 GT overheating, If it's not the fan, or the thermostat

What is your question?

I took it to get a tune up as soon as I bought it, The mechanic there proceeds to tell me that he had the car running and it started to overheat. He tells me he thinks it's the head gasket. I buy some head gasket sealer and run it through the system it doesn't help. I then check the thermostat, it opens and closes just fine. It doesn't seem to be using much water and there is no milkiness to the oil. The van cuts on and off just fine. There is a hissing sound coming from somewhere around the block when it starts running hot and I seen what look like white smoke coming out of a hose that is connected into the purge valve canister. I leave the heater running wide open, it doesn't seem to slow the problem. The check engine light and another light is on. When you first start the car at AutoZone itself up real high, and then drops down real low.

What makes this problem better or worse?

Driving anything above four or five miles and it runs hot.

How long has your 2005 Pontiac G6 had this problem?

3 weeks