I recently purchased a 6750 watt generator when our power was out for 3 days. It worked everything well except for the gas furnace. The furnace initiated and then gave an error code related to the voltage. When measuring the voltage across the generator I get about 140v but also can measure 50v between the ground and neutral. I don't get it. Any ideas

I can tell you the voltage outputs are extremely out of proper tolerance and that caused the furnace error code. I am a building and utility electrican and cannot speculate as to the problem with the generator, but it clearly needs servicing or repair by an authorized generator dealer or repair shop.