Russia is not manufacturing more weapons than its GDP suggests. Its simply spending more money than the west donates to Ukraine (Because Russia is fighting for Putin's survival, while its not #1 priority in the west), and importing everything from China using that money.

Russia is actually precisely the example why GDP matters. Russia's position before the war was an overwhelming advantage, pinching Ukraine from three directions in a huge concave, in a flat plains basically without geographical barriers, against a divided, poor, and old (unlike say Afghanistan which has endless 18 year old men) country in Ukraine.

Yet it has fought the war into a extremely expensive stalemate, with draft imposed, large casualties, and basically total loss of international prestige.

All that, because its low GDP meant no high end equipment, no advanced logistical capabilities, no sophisticated information advantage. While, the US, by Elon Musk simply donating some starlinks (A definitive high GDP product), made all Russian attempts at disrupting Ukrainian command chains useless.