They don't cover the % of revenue derived from overseas. Ie the GDP of the US is less meaningful over time for the S&P 500. It really should show total market cap / GDP of relevant jurisdictions weighted.
I don't think you are wrong, but then some of the US GDP belongs to other countries so there are also some deductions to do.
Yup, I guess the MSCI 1500 (or similar) / global GDP is probably easiest.