Gamma exposure (GEX)

When a trader buys or sells an option, a market maker usually takes the other side and then hedges that position in the underlying. How much they must buy or sell as price moves is governed by gamma — the rate at which an option’s delta changes. Aggregate that across every open contract and you get a map of how much hedging pressure sits at each strike.

The sign matters more than the size. When dealers are net long gamma, hedging works against the move: they sell into strength and buy into weakness, which tends to dampen ranges. When they are net short gamma, hedging follows the move, which tends to amplify it.

GEX is a model, not a measurement. Dealer positioning is not published; it is inferred from open interest and assumptions about who is buying and selling. Treat it as a description of likely hedging behaviour, not a forecast of direction.

Hermes computes position GEX from a live option-chain snapshot for any optionable US ticker, defaulting to every expiry inside a 90-day window, and publishes the resulting levels on each ticker’s public page. SPX and SPY additionally carry intraday flow GEX built from same-session options trades.