This is an extreme lower-bound estimate as the real GDP includes factors not accounted for here like capital formation and offchain demand for ETH. Countries with larger GDPs can issue and maintain a higher valued money. the us m2 is 22T and GDP around 30T. A low growth / restricted nation has a low multiple like Iran with an m2 of 175b USD and GDP of 430b. A high growth example is malaysia with a GDP of 444b and m2 of 556b. A rough estimate of Ethereum cGDP is onchain fees + capital formation/deployment (onchain RWA+ defi TVL). That puts ETH around 300b cGDP with a reasonable growth multiplier to get our ETH MCAP of 511b.