Visited the link you posted. There are so many different GARCHs, which one should I use if I want to test SPY?
You're over-thinking it, FeChief. A tailored, general(-ized) AutoRegressive ("on traded prices) model better represents the look-ahead fear/confidence of the market (and thus, direction) than the defined Historic Volatility (±1σ of price), but not quite so well as the market's OWN look-ahead fear/confidence measure, "IV".