Zero Gamma in one sentence
Zero Gamma is the spot price at which the net gamma of options dealers is zero. Above it, dealers are net long gamma and their hedging pushes against price moves; below it they are net short gamma and their hedging pushes with them.
That is why the same index behaves like two different markets depending on which side of Zero Gamma it trades: calm and mean-reverting above, fast and trending below.
Where the level comes from
Market makers who sell options hedge them by buying and selling the underlying so that their book stays delta-neutral. How much they have to trade for a given move is their gamma. Every strike in the option chain carries some gamma, weighted by its open interest, and each strike's gamma changes sign for the dealer depending on whether the position is a call or a put.
Add all of it up across every strike and expiration and you get the dealers' net gamma at the current price. Move the price up or down and that sum changes. Zero Gamma is simply the price where the sum crosses zero. GexVision computes it on the full SPX, NDX, SPY and QQQ chains and refreshes it every five minutes during the session.
What changes above and below it
Above Zero Gamma (positive gamma regime): when price rises, dealers sell; when it dips, they buy. Their hedging is a brake. Ranges tend to be narrower, pullbacks get bought, and intraday moves fade more often than they extend.
Below Zero Gamma (negative gamma regime): when price falls, dealers must sell more; when it rises, they must buy more. Their hedging is an accelerator. Ranges widen, moves overshoot, and reversals are sharper. Most of the big down days happen in this regime.
How traders read it
As a regime line, not as a buy or sell signal. The first question of the day is which side the index opened on. The second is whether it crosses during the session: a break below Zero Gamma with follow-through is the moment the tape changes character, and a reclaim is often where a sell-off runs out of fuel.
It works best together with the Call Wall and Put Wall, which frame the likely range, and with the size of the gamma itself: a Zero Gamma with a lot of gamma around it matters more than one in a thin part of the chain.
What Zero Gamma is not
It is not a forecast and it does not know where price is going. It describes how the hedging of one large group of participants will react to a move, which is useful precisely because it is mechanical.
It also moves. Open interest changes once a day, but the price does not sit still and 0DTE options add and remove gamma within the session, so the level you see at 9:30 is not always the level at 15:30. GexVision keeps the opening levels next to the live ones so you can see both.