1: https://tradingeconomics.com/european-union/gdp-per-capita
(USGDPPC - EurozoneGDPPC) / AVERAGE(USGDPPC, EurozoneGDPPC) = 0.283
Roughly speaking, you could write this as "The GDP per capita of the Eurozone is 28.3% lower than the US".
There's no single apples-to-apples measurement we can make; the US has more natural resources than the EU, suffered far less harm from all the major conflicts up through WW2, &c.
I don't know whether or not liberalizing the economy of the EU would raise per-capita GDP or not, but the post I was replying to was claiming that a very specific and easy-to-check fact was wrong, so I checked it.