According to Wikipedia The G20 (or G-20 or Group of Twenty) is an international forum for the governments and central bank governors from 20 major economies.
Central banks control a country's money supply, and interest rates. It's important to note that none of the currency of G20 members has a gold standard, it's all just fancy paper.
The G20 protesters were left wing anti-capitalist extremists, who didn't get the memo that central banking is far from a capitalist practice.
Capitalism is an economic system in which trade and industry are controlled by private owners for profit, rather than by the state (Doesn't seem like G20 is very compatible with capitalism does it?).
Central banking is neither a capitalist nor socialist, but an authoritarian system created as a way to control the economy.