Need to maintain gelato at 10-18 °F without electricity
You could experiment with salt to ice ratio. Somewhere around 15% salt to ice ratio you get freezing in 10-18° F range. 

Once you are at 10-18° F, you should have extremely good insulation to keep the temperature stable, have a thermometer placed very close to your gelato and constantly check that, have lots of frozen ice+salt mixture and minimized the exposure of your frozen content to ambient environment.