Are you intimately familiar with the inner workings of your heatpump? Nearly all heatpumps in a cold climate have backup heat built in and it would automatically switch to backup when it gets too cold outside. -30C is well into the too cold category for it to function as a heatpump alone
Home generators are not very efficient, they lose a lot of energy to heat and noise. It’s close to the break even point so depending on your generator and heatpump, you could use more gas than with a modern gas furnace. Even with a theoretical perfect 100% efficient generator, it’s not going to be an order of magnitude less gas because heat pumps are not 1000% more efficient than a gas furnace
Are you intimately familiar with the inner workings of your heatpump? Nearly all heatpumps in a cold climate have backup heat built in and it would automatically switch to backup when it gets too cold outside. -30C is well into the too cold category for it to function as a heatpump alone