Gas alone does not produce electricity. Gas power stations do. This means building enough gas power stations to cover power requirements of the entire country, then having them sitting most of the time idle there, "just in case".
This is the core of the problem: wind and solar need to be massively backed up. You cannot rely on them supplying even 5 per cent of their nominal capacity if the weather turns against you.
Such a massive backup is eye-wateringly expensive, too. And it consumes concrete and steel, two elements whose production is carbon-intensive.
If you assume:
* We dont already have the infrastructure.
* Power cant be imported.
* Power usage cant be timeshifted.
* Power cant be stored in batteries.
* 10 day periods without wind and sun are routine.
Then yes, it might get expensive.
https://iopscience.iop.org/article/10.1088/1748-9326/ab91e9
In your once in a lifetime event I'd be happy if we used a bit of gas. This is something, after all, that Japan also had to do for years after Fukushima went kaput.
Bear in mind also that demand for heating and transportation can be timeshifted. An Octopus tariff in the UK already allows for this. We've barely scratched the surface of whats possible here because electric grids are slow to evolve to this new reality.