A 7% average annual return over 24 years is not necessarily bad. On an absolute average basis that trails the market by 3%. [3] That return could still be legitimately superior to investing in SPY, depending on risk goals, if its volatility and beta exposure is sufficiently low that it beats SPY on a risk-adjusted basis. Then you may even be able to beat the S&P on a total basis, safely, with leverage, depending on the volatility ratio.
Therefore the point you're trying to make would be better construed with an analysis of the beta, volatility and Sharpe of GMWAX. By definition the beta of SPY is 1, and the historical Sharpe is around 0.72. It's correlation with the market is about 1 and it has (again, almost by definition), 0 alpha.
GMWAX has close to total correlation with the market, with a beta of 1.02 and negative alpha less than that of SPY. It has a Sharpe ratio of only 0.34. That means this GMO asset allocation fund is not beating the market, even on a risk adjusted basis. Moreover the explicit mandate of the fund, to avoid market downturns but otherwise track the market's upswings, is not being met. It is both underperforming the market and highly correlated with the market.
1. https://www.schwab.com/public/schwab/investing/investment_he...