My GW-2078 fireplace fan does not seem to be working and therefore, each time I try to turn on the heat, it shuts itself off. Have you had experience with this & how to fix/what part to replace?

Sometimes its as simple as WD40 on the ends of the motor where the spindle is held in place by the metal. I have a forced air fireplace and I would service it every year with getting into the motor compartment and cleaning this out and oiling. The get a lot of dust in them every year. Try, after the unit is unplugged, to manually spin the fan. It should spin very easily. If not just do some cleaning and try the WD40. I have had mine for roughly 18 years and it still works because I do this type of service. Any questions, feel free to ask.