Was it smoking as in smoke---Or steam?
If I can suggest having someone do a pressure test on the cooling system and finding out if there is a leak in the system and where--a pressure test in some cases can point to the right direction.
As for other possible causes? There are of course varying degrees of problems. It could very well be the water pump, and yes--Head gasket can also be suspect. It's impossible to tell without doing detailed systematic troubleshooting.
I would start with a pressure test on the cooling system and see if a) the system holds pressure. b) The system drops in pressure=fluid loss at some point.
I would also recommend full inspection of the thermostat and water pump. Water pump is highly suspect in why it would overheat, as it's my understanding they used a plastic impeller pump. If it's smoking or "Steaming" that points towards something letting loose that a pressure test might help pinpoint.
S-