Depending when you begin averaging GDP growth, from the figures of the actuaries at the Social Security Administration, we get an average GDP growth of about 1.5%. Actually, their GDP growth is a shrinking figure, ending up at 1.2% in 2075.
Anyhow, if total return historically is roughly twice GDP, Seth Ackerman's take sounds about right to me. It is total return we are talking about.
As to why stock market total returns are twice GDP growth, I pass.
-- Homines id quod volunt credunt.