My 2003 Chevy Malibu dies after i start it.?

[alanaefird]

I just replaced the head gasket and thermostat. My car was over heating way too much. Now, the RPM goes up and down by itself. when i put it in park, it shoots up to 2. The temperature guage doesn't move. It stays on 100 all the time. Use to it would be at 200 instead of 100 all the time that was normal since i bought the car. Now, after replacing the head gasket it loses power while im driving or trying to leave and has started overheating again but the temp guage says 100.
what's making it die and why isnt the temp guage reading the right temperature.