There is no battery capacity required for the simple effect of substituting gas power plant operation. Any MWh generated by another source will simply remove the need to generate that MWh from gas, where the price of gas is the dominating component.
Not really. This is overly simplistic since it's not accounting for reliability/predictability of supply. Natural gas can be turned into MWh on demand, at night, when there's no wind, during consumption anomalies, etc. It's clearly not interchangeable with renewables MWh for MWh like you're suggesting.
You also have to pay the gas. Gas backups for power are just stupid without any own supply.
In winter, we also need the gas for heating, that will not change in the next 30-40 years.