Good call. Replace the thermostat, they are cheap (~$10). You can test it by pulling it and putting it in a pot of water on the stove. Bring it to a boil, and you should see it open up just before the water boils, at around 180F. Alternately you could try running the car without the thermostat and see what happens.
Are the radiator hoses plugged/collapsed? How is the radiator?
Was the block and head checked for levelness before the head gasket was installed? If the gasket blew, there is a possibility that the head or deck could be slightly warped causing a bad seal in the new gasket. Same could be said if the old gasket wasn't fully removed.