What gamma exposure measures
Every option has a delta, how much its price moves per point of the underlying, and a gamma, how fast that delta changes. A market maker who sold you an option hedges its delta with the underlying. When price moves, the delta changes and the hedge has to change with it. Gamma exposure is the size of that adjustment: how many dollars of the index dealers must buy or sell for a 1% move.
For a single strike the number is gamma × open interest × 100 shares × spot² × 1%. Add it up across the whole chain, with the sign flipped for positions where the dealer is on the other side, and you get the dealers' net GEX at the current price, in dollars per 1% move.
Why it moves the market
Dealers do not have a view; they have a book to keep flat. If they are net long gamma, a rally leaves them long delta and they sell; a dip leaves them short delta and they buy. Their flow leans against price and dampens it. If they are net short gamma, the same rally forces them to buy and the same dip forces them to sell. Their flow chases price and amplifies it.
In SPX and the big ETFs the notional involved is large enough that this hedging is a visible part of the intraday tape, especially near the close and around heavy strikes.
Positive and negative GEX
Positive GEX environments are the calm ones: narrower ranges, dips bought, realized volatility below implied. Negative GEX environments are the fast ones: wider ranges, gaps, sharp reversals, and most of the large drawdowns. The border between the two at any moment is the Zero Gamma level.
Magnitude matters as much as sign. A small positive number with the index far from any heavy strike is a weak brake; a large positive number with price pinned under a heavy call strike is a strong one.
Reading the GEX profile by strike
Plot the GEX of each strike and you get a profile: tall bars where the market is positioned, empty space where it is not. The tallest call bar above price is the Call Wall, the tallest put bar below is the Put Wall, and the largest bar in absolute terms is Max Gamma. The share of gamma that expires today (0DTE) tells you how much of that structure will vanish at the bell.
GexVision computes this on the full chain of SPX, NDX, SPY, QQQ and a list of large stocks, refreshes it every five minutes during the session, and turns it into levels you can put on a TradingView or Bookmap chart.
The limits, stated plainly
GEX assumes a dealer positioning convention (dealers long calls and short puts, or a variant of it); real books differ, and the model is an approximation. Open interest is published once a day, so the chain used during the session is yesterday's positioning plus today's price. The data GexVision uses is delayed about 15 minutes.
None of this is a recommendation. GEX describes a mechanical flow from one group of participants, and that description is useful precisely because it does not try to predict anything else.