What is options gamma exposure (GEX)?

Gamma exposure (GEX) is an aggregate measure of the hedging obligations that options market makers (dealers) face across all outstanding options contracts on a given underlying, most commonly the S&P 500 or individual equities. When a dealer sells an option, they must continuously delta-hedge by buying or selling the underlying asset as prices move. Gamma determines how much that hedge changes for each point of price movement.

When aggregate gamma exposure is positive (dealers are net short calls), price increases cause dealers to sell the underlying (their delta hedge grows shorter) and price decreases cause them to buy. This creates a stabilizing, mean-reverting force that pins the market in a narrow range and suppresses realized volatility. Options expiration dates with large open interest often create "gravity" around strike prices with high gamma.

When gamma exposure turns negative (dealers are net short puts, common during selloffs), the dynamic inverts. Price declines force dealers to sell (their hedge demands more short delta), and price rallies force dealers to buy, amplifying moves in both directions. This negative gamma regime is associated with the outsized intraday swings typical of volatile market selloffs.

Traders and systematic strategies monitor GEX estimates to gauge the volatility regime. The transition from positive to negative gamma often coincides with notable increases in realized volatility. While GEX calculations rely on assumptions about dealer positioning and cannot be directly observed, the framework explains many empirical patterns in equity market microstructure, including why volatility clusters after options expiration strips large open interest from the market.