GEX and the Gamma Flip: What Gamma Exposure Can and Cannot Tell You

GEX estimates whether dealer hedging will dampen moves or amplify them.

Quick refresher. Gamma is how fast an option's delta, the size of the dealer's hedge, changes when price moves one point. A dealer is the market maker who takes the other side of customer options. Own an option and you are long gamma; sell one and you are short gamma. Open interest (OI) is how many option contracts are still open at a strike. The index in the examples is the S&P 500 (SPX), whose future is ES, and one SPX option moves 100 dollars for every index point: that is the "x 100" in the formula.

A dealer who is long gamma sells rallies and buys dips, which leans against price. A dealer who is short gamma buys rallies and sells dips, which pushes price further the way it is already going. It never starts a move. Add up every strike's dealer gamma and you get one number: positive suggests dampening, negative suggests amplifying. The price where the sum changes sign is the zero-gamma flip, also called the gamma flip.

Naive modelThe GEX reading on this page is the naive version, from the 2017 SqueezeMetrics paper that popularised it. The maths underneath is marked Fact. It builds the right intuition. It is not the literal market: dealers are not the only counterparty, and nobody can see real dealer inventory.

Fact marks Black-Scholes maths and arithmetic. Naive model marks an assumption about positioning or what price will do.

• MechanicsDealers are the counterparty and hedge to neutral. They take the other side of customer options and trade the underlying to stay flat. This part is sound.
• AssumptionDealers are long the calls and short the puts customers trade. So call gamma counts plus, put gamma counts minus. This is a guess about positioning, and it is the one that fails first. The gamma itself is identical for a call and a put at one strike: only the assumed side differs.
• MeasurementOpen interest stands in for dealer positions. The arithmetic is exact. Treating open interest as the dealer's book is part of the guess. Per strike: gamma x open interest x 100 x price squared x 0.01 = gamma exposure per 1% move. Sum every strike for net GEX. Open interest is tallied after the close, so it is last night's book.
• InterpretationPositive GEX dampens, negative GEX amplifies. Positive: dealers sell rallies and buy dips, a quieter, rotating tape. Negative: they chase, a trending, faster one. The flip is where the regime is assumed to switch.

A made-up book: open interest bunched around 6,000, calls on the strikes at and above it, puts at and below it. Each bar is that strike's gamma exposure at the price you set, using the formula above with the Black-Scholes gamma of one contract, signed by the naive assumption. Drag the price and watch bars near it grow and far ones shrink.

FactNotice the flip does not sit where the calls start. It sits where the sizes balance. A real board computes its flip from a live book with its own method, and it moves as price, vol and positions change: take the method from here and the level from your board.

FactThe arithmetic is exact. The inputs are estimates. Start with a worked example: index 7,700, open interest 20,000, gamma 0.0012 per contract.

FactThe unit is a choice. Other boards print per point, or in contracts. At 6,000, per 1% reads sixty times bigger than per point. Rank bars against each other on one board, and never carry a size threshold from one platform, product or unit to another.

Naive modelA GEX reading can suggest something about the character of the tape. It can never tell you which way. Sort the claims.

Naive modelI treat GEX as a setting on my risk dial, not as a trade. Positive, I expect more rotation: smaller targets, and I am more willing to fade. Negative, I widen targets, stop fading and respect breakouts. It never tells me which way.

I hold it loosely, because the sign underneath it is a guess that can be backwards on any given day with no warning, and the open interest behind it is last night's number. And I never read "dealers are long gamma here" as a measured fact.

Reading gamma exposure honestly is one piece of reading the market. The Mentorship is where you put the whole process together.