It won't change much for Russia. When there's a seller, there's always a buyer. If not at this price, there will be at a lower one. Some countries will buy Russian Gas. For instance, Orban said he's not going to stop trading with Russia because Hungary cannot afford it.
Many countries have voted to stop trading with Russia but they have contracts until the end of this year, so they will keep buying until 2023. I doubt the war will last that long. It will either end with Russia falling back after getting assurance from Ukraine that it won't join NATO (which it is already ready to give), or escalate to a war with the EU and NATO. This isn't too improbable because almost 50% Russians support the invasion of Ukraine and blame the EU for sanctions. Most supply chains from the EU to Ukraine go through Polish border. I wouldn't be surprised if Putin shot a few missiles towards the border which would be a direct attack that NATO might not let slide.