It could be a bad governor assembly. It is driven by the output shaft. It senses road speed and controls shifting.

I just installed a fel-pro head gasket set in my GST. The materials are definitely different. For example the oem throttle body gasket was steel and the fel-pro was some type of composite. The thermostat housing gasket was incorrect, but I used RTV anyways. A second water pump o-ring is not...

So the starter turns the engine, but the engine never takes over? Was the work done by a mechanic, or did you do it yourself? A little more information would be helpful.