If as you said that the Russian economy depend so much on the sales of gas and oil, then I think banning the sales of gas to EU countries will do Russians more harm than good. China will definitely take advantage of this situation since they are now the major buyers.
However, it won't be easy on the European countries as they now have to seek other alternatives to Russian gas, this will affect their economy as well. But as it's been said, necessity is the mother of innovation. They have been surviving without Russian gas and oil.
No one is saying that the EU countries will easily abandon Russian oil and gas. First of all, long-established logistics supply chains suffer. These are additional problems for both Russia and the European Union. However, Russia began this confrontation and open blackmail of Europe, and therefore Europe decided once and for all to get rid of such blackmail. In the first period, it will be more difficult for Europe, but subsequently Russia will suffer much more from the rupture of its oil and gas supplies to the EU countries. It was very convenient to make such deliveries through the oil and gas pipelines of Russia, to do it this way.
by ships and barges to the other side of the world, it is much more difficult and expensive. If Russia still finds a worthy replacement for consumers.