1) Gas peakers - where every kilowatt hour delivered by solar or wind is just a kilowatt hour of gas that would otherwise have been burned. We are generally still here - still burning gas while it's sunny and windy.
2) Pumped storage and batteries gets us to 98% carbon free grids with ~5 hours of storage with 90% roundtrip efficiency - https://reneweconomy.com.au/a-near-100-per-cent-renewables-g...
(98%/5 hours is for australia and will vary for different countries but probably not wildly).
3) Syngas fills in that last 2-5% with ~50% roundtrip efficiency. Every kilowatt hour used in those 5% times - those dark, windless nights will be quite expensive although, counterintuitively still cheaper than an every kilowatt hour generated by a nuclear power plant - https://theecologist.org/2016/feb/17/wind-power-windgas-chea...
3 and to some extent 2 will require natural gas to be prohibited or taxed heavily.
Just one note, I believe what you mean is some form of gas made from renewables, most likely hydrogen.
"Syngas" is a term that has a relatively specific meaning in the chemical industry, notably it is a gas mixture of mostly Carbon Monoxide and Hydrogen. I do not think that this is what you mean.