I think the evidence being cited does not support the statement implied, namely that "GDP growth does not impact [or negatively impacts] stock returns"
If you are talking about the entire market, then it is blindingly obvious that higher GDP growth, all else equal, equates to higher stock returns. But as I posted earlier, in addition to GDP growth, there is also (a) dividend yield, basically, what rate of capital reinvestment is required to generate the GDP growth, (b) corporate profit margins and tax rates, and © valuation multiples. Those are, mathematically, the four key forces influencing pre-tax, whole-market stock returns.
So I guess the argument is really that there is a certain relationship between GDP growth and the remaining three variables that is causing GDP growth and stock returns to not have the same positive correlation one would see if they held fixed all other factors.
But to me, that doesn't seem to be important in understanding the point of this topic. The point was, will the returns of the whole market [presumably, the whole U.S. market] be lower in the future than over the past 50 years. If one believes that profit margins, tax rates, and valuation multiples are unlikely to be more favorable in the future than today, and that GDP growth is likely to be lower as well, then this conclusion follows mathematically regardless of the studies being cited.