GEX Explained: Gamma Exposure in NIFTY & Bank Nifty Options

The one idea behind GEX

A market maker who sells you a NIFTY call is short that call. To stay directionally flat they buy futures against it. How much they buy is delta. How fast that required amount changes as spot moves is gamma.

Gamma is therefore a hedging-flow multiplier. It says: for every point NIFTY moves, the dealer must trade this much more index to stay flat. Aggregate that requirement across every strike on the chain, weight it by the open interest sitting there, and you get gamma exposure — GEX, the net amount of index buying or selling that dealers are mechanically forced to do per unit move.

The critical part is the sign. Because it flips the direction of that forced flow, and with it the entire character of the session.

Positive gamma: the market pins

When dealers are net long gamma — the usual state when retail and institutions are heavy net sellers of options around the money — their hedging is counter-trend. Spot rises, their net delta rises, so they sell index to get flat. Spot falls, they buy. Every move is met with a hedging flow that pushes back.

The observable result is compression. Ranges narrow, breakouts fail at the edges, and price gravitates toward the strike with the largest gamma concentration. On expiry days this is the mechanism behind pinning: price is magnetised to a strike not because anyone wants it there, but because the aggregate hedging book keeps pulling it back.

• Ranges are tight and mean-reverting; fading the extremes works.
• Breakouts fail more often than they run — require a decisive close, not a wick.
• Realised volatility undershoots implied volatility; option buyers bleed theta.
• Price clusters at the highest-gamma strike, especially into expiry.

Negative gamma: the market accelerates

When dealers are net short gamma — typically after a period of heavy option buying, or after a large directional move has shifted the book — their hedging becomes pro-trend. Spot falls, their net delta falls, so they must sell more index to stay flat. That selling pushes spot lower, which forces more selling.

This is the feedback loop behind vertical moves. Nothing fundamental has changed; the hedging book has simply flipped from damping the market to amplifying it. Negative gamma sessions produce the gap-and-go days, the 3 p.m. air pockets, and the trend days that punish every mean-reversion trader in the market.

• Trends persist; fading the move is how accounts die on these days.
• Realised volatility overshoots implied; option buyers finally get paid.
• Support and resistance break cleanly rather than holding.
• Volatility begets volatility — the move accelerates rather than fading.

The gamma flip point (zero gamma level)

Between those two regimes is a spot price at which aggregate GEX crosses zero. Above it dealers are typically long gamma and the market pins; below it they are short gamma and the market accelerates. This is the gamma flip, also called the zero gamma level.

It is the single most useful number GEX gives you, because it converts an abstract regime into a price on your chart. Above the flip, trade the range and sell the extremes. Below it, respect trend and stop fading. When spot is oscillating around the flip, expect the character of the tape to change hands repeatedly — that is exactly what a regime boundary looks like from the inside.

How GEX is computed from the option chain

The calculation is mechanical. For every strike on the chain you need the gamma of the option and the open interest sitting at that strike.

• Compute gamma per contract for each strike using a Black-Scholes gamma with the correct time to expiry.
• Multiply by open interest at that strike and by the contract multiplier (lot size).
• Multiply by spot squared and by 0.01 to express the result as index units traded per 1% move.
• Sign the result by assumed dealer position: dealers are conventionally treated as long calls and short puts against retail flow, so call gamma adds and put gamma subtracts.
• Sum across all strikes for total GEX; keep the per-strike values to find the largest gamma concentration and the flip point.

Two inputs decide whether the output is meaningful: the volatility assumption and the time to expiry. Both matter most on the final day, when small changes in either move the gamma profile substantially. Published GEX figures vary widely between sources for exactly this reason, so treat any single number as one estimate rather than a measurement.

Trading with GEX, practically

GEX is a regime filter, not an entry signal. It tells you which playbook is live today; your entry still comes from price structure — the previous day high and low, the opening range, VWAP, an OI wall.

The highest-quality setups appear when GEX and price structure agree. A positive-gamma session with spot pressing into the largest gamma strike from below is a high-probability fade. A negative-gamma session breaking the previous day low with expanding order flow is a high-probability continuation. Alignment is the edge; disagreement is a reason to stand aside.

• Check the sign of total GEX before the session opens — it sets today's playbook.
• Mark the gamma flip level on the chart as a regime boundary.
• Mark the largest-gamma strike as a magnet, especially on expiry day.
• Re-check after any large move; a 1.5% index move can materially reshape the book.

Limits and honest caveats

GEX rests on an assumption about which side the dealers are on. That assumption is a convention, not observed data — exchanges in India do not publish dealer inventory. When large institutional flow runs the other way, the sign can be wrong.

GEX also inherits the batched, delayed nature of open interest. A chain that refreshes every few minutes cannot give you a tick-accurate flip point, and intraday OI changes are not attributed to dealers versus end users. Treat GEX as a strong prior about the character of the day, cross-check it against realised behaviour on the tape, and drop it the moment the market is plainly disagreeing.