Seven percent is the historical return of the US stock market. Other markets have different historical returns. This "comes from" the historical data of US stock prices. Comparison to GDP or inflation rates don't apply.
You said "Seven percent is generally accepted as the expected market return over a period of time sufficiently long enough to mitigate systemic risk, ". Also, When people say "historical return is 7%", they do not say it as a historical fact, but as a forward expectation like you did.
So, you need to provide why 7% is generally accepted, and why not 2%, or 70%.
Comparison to gdp and inflation definitely apply. Business profits in an index grow with Inflation and gdp growth, and stock value is the expectation of future profits discounted over time.