You're right about my assumption that off-grid = solar + storage. Not only was that the focus of the analysis on which I was reporting, but it is the combination I most often see in reference to going off-grid. It's also perceived as being universally available in the sense that the sun shines everwhere, aside from major differences in annual insolation, while not every home is connected to the gas "grid".
Your gas-generation option has other advantages, besides price. Because interruptions in gas supply are much rarer than power outages, let alone the cycles of PV cells, storage would only be necessary if peak instantaneous load were greater than the maximum output of the generator. That would allow storage to be sized differently: not just smaller but perhaps with a different chemistry or even based on ultracapacitors that excel at high discharge rates.
I also agree that choice entails trade-offs. Even the natural gas fuel cells marketed by high-profile companies with sterling green images may emit more CO2 per kWh than today's most efficient combined-cycle plants.