Gamma Exposure (GEX) measures how much market makers must buy or sell the underlying when the price moves by $1. It is computed from options chain data (open interest and implied volatility) using Black-Scholes gamma across the nearest 6 expirations. Positive GEX (green) = dealers buy dips and sell rallies, stabilizing price. Negative GEX (red) = dealers sell dips and buy rallies, amplifying moves. The Call Wall is the strike with largest positive gamma (resistance). The Put Wall is the strike with largest negative gamma (support). Data covers S&P 500 + Nasdaq 100 (~520 stocks). End-of-day only (OI updates once daily).