GDP is almost irrelevant when it comes to Russia's true strength and persistence.
It's true however that winning a war is incredibly difficult, and the outcome could still be the same for Russia, if their GDP was increased by an order of magnitude.
Russia also has a unique ability to turn the entire economy into a sort of "war economy". That hasn't really been done yet.
Russia’s ability to implement a war economy isn’t unique - the last time they did it (WWII) so did the UK, Germany, and the US.
More recent examples include the both sides of the Iran-Iraq war and Armenia in its war with Azerbaijan.
GDP has implications for the productivity of an economy and the budget of the government.
41% of Russia’s government spending at the minute is on the war which means they have to either cut spending on other things weakening the state or drive massive debt and inflation.
There’s a reason you can only run a war economy for so long - eventually you win the war or bleed your economy dry. Russia’s had the advantage of huge legacy reserves of equipment from the USSR days and a larger population. We’ll see how long they can last tapping the well.
It's true however that winning a war is incredibly difficult, and the outcome could still be the same for Russia, if their GDP was increased by an order of magnitude.
Russia also has a unique ability to turn the entire economy into a sort of "war economy". That hasn't really been done yet.