It's not that the grid is reliant on gas, it's that if people switch from gas to electric for heat quickly, the grid might not be able to handle the sudden extra load.

Either way, it is known how much gas is available. It can also be estimated fairly reliably how much gas will be used a couple of days in advance, given the weather forecast. Therefore a gas shortage will be known in advance, and contingency measures will be taken, both for gas and electricity.