GDP is estimated with a fair degree of accuracy in G20 countries... but those same countries have very old demographics (lower growth). In most of the world GDP is at best a rough guess. Black markets and barter systems dominate more than half the globe, while VAT taxes encourage under-reporting throughout the EU.

Many industries have been nationalized in countries that are supposedly private capitalism. The US government controls 95%+ of the US mortgage market, while government sponsored entities (utilities, telecom, money center banks, defense contractors, universities, hospitals) are officially private but are completely controlled by government... these entities tend to over-report their GDP contributions. Bureaucrats the world over are judged by how much money they spend (or report spending), never for efficiency.

GDP numbers may be significantly over or under estimated— and as the share of private vs public economic activity changes, the measuring error is not consistent even within a given country.

In G20 GDP estimates, the noise is almost as big as the signal. In emerging markets (where most of the growth is today) the noise is bigger than the signal. This makes market cap to GDP ratios highly inaccurate— maybe too high, maybe too low— and over time probably both.

An economic model is only as good as the data going into it, and GDP is poorly measured.