In 2021, Germany got 55% of its gas from Russia (the rest from other European countries). Depending on how much overall consumption can be reduced, European suppliers might be able to cover most/all of it.
It's also not as easily replacable as "Just go to the other gas supermarket down the street", infrastructure needs to exist and support such supply chains.
[0] https://www.statista.com/statistics/1201743/russian-gas-depe...
"Habeck said imports of Russian oil now accounted for 25% of German imports, down from 35% before the invasion, and gas imports have been cut to 40% from 55%. Russian hard coal imports were down to 25% from 50% before the invasion."