Oh I thought we were talking about generating natural gas....I was going to suggest a case of Natty Light and a 30 pack of sliders. Nevermind.
It also depends on your mission.
I have enough gasoline stored to power my generators through their service lives. But I am also off-grid, my house is designed to be off-grid, and my power requirement for the entire house and shop is a fraction what the typical home is. And if you walked into the house you'd never know it is off-grid.
So even without the solar panels, I could burn only about a gallon a day of fuel and not notice any difference.
But that is a very different mission than keeping an all-electric house operating normally through short term outages. For that a NG or propane fueled backup generator is preferrable, because the generator there will be burning through 5-10 gallons of fuel a day. No big deal for a weeklong power outage, but a really big deal for long term survivalism.
For intermediate term use, a battery bank and inverter-charger that is hooked to critical circuits in the house and linked to a backup generator is the optimal solution. Normally the grid keeps the batteries topped off which drastically increases their life. But if the power goes out you have a few hours to deal with getting the generator going. And then when the generator is going you only have to run it a few hours a day to charge the batteries back up and power circuits at the same time. This is MUCH more efficient than having a generator running 24/7.