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Dealer Gamma Exposure: What GEX Can and Cannot Tell You

For educational purposes only; not investment advice.

Direct answer

Section titled “Direct answer”

Dealer gamma exposure (GEX) is an estimate of how the aggregate Delta of option dealers may change when the underlying price moves. If dealers hedge Delta dynamically, their Gamma sign can imply conditional trading pressure: a long-Gamma dealer generally sells underlying after a rise and buys after a fall, while a short-Gamma dealer generally buys after a rise and sells after a fall.

GEX is not a directly reported dealer position. Public open interest shows outstanding contracts, not who owns each side, whether the position is customer or dealer inventory, or how it is hedged with stock, futures, other options, or OTC instruments. A GEX chart is therefore a model output built from market data plus unobservable-position assumptions, not a deterministic price signal.

Estimation and hedge direction

Section titled “Estimation and hedge direction”

For contract i, a simple share-equivalent sensitivity is:

Delta change per $1 move ≈ Gamma_i × assumed dealer sign_i × OI_i × multiplier_i

A widely used dollar-notional convention for a 1% underlying move is:

GEX_1% ≈ Σ(Gamma_i × assumed dealer sign_i × OI_i × multiplier_i × S² × 0.01)

The second expression multiplies the estimated change in hedge shares for a 1% × S move by spot S. It is a sensitivity or notional estimate, not a forecast of actual dollars that must trade. Vendors may omit S, the 0.01, or use different sign conventions, so numbers are incomparable until their units and assumptions are aligned.

Long Gamma means option Delta increases as spot rises. A Delta-neutral long-Gamma dealer would normally sell more underlying on a rise and buy it back on a fall, a countertrend flow. A short-Gamma dealer does the reverse, a procyclical flow that can amplify movement. Actual rebalancing depends on hedge thresholds, liquidity, transaction costs, other Greeks, inventory netting, and the dealer’s risk policy.

One strike, two possible conclusions

Section titled “One strike, two possible conclusions”

Suppose spot is 100, model Gamma is 0.02 per $1 move, open interest is 10,000 contracts, and the multiplier is 100.

0.02 × 10,000 × 100 = 20,000 share-equivalent Delta change per $1 move.

Because a 1% move from 100 is $1, the common dollar-notional version is:

0.02 × 10,000 × 100 × 100² × 0.01 = $2,000,000 per 1% move.

If dealers are assumed long those options, a rise from 100 to 101 would imply roughly 20,000 shares of incremental selling to restore Delta neutrality; if dealers are short, the same inputs imply roughly 20,000 shares of buying. If only 40% of the open interest belongs to that dealer-side position, the estimate falls to 8,000 shares. The public OI number alone cannot choose among these cases.

Gamma also changes as spot, time, and implied volatility change. Near-the-money contracts close to expiration can produce high local Gamma, but their exposure may disappear, move to a different strike region, or be closed during the session. Recalculate rather than treating yesterday’s “Gamma wall” as fixed.

Research checklist and limitations

Section titled “Research checklist and limitations”
  • Record the timestamp, spot, forward or dividend assumptions, volatility surface, rates, contract multiplier, exercise style, and expirations included.
  • State how dealer signs are inferred. Rules such as “dealers are short all calls and long all puts” are assumptions, not observations.
  • Separate open interest from same-day volume. OI is updated with a lag and does not disclose intraday opening versus closing trades.
  • Include futures, ETFs, related index options, and OTC positions only when data supports them; otherwise label the omission.
  • Recompute Gamma across spot scenarios. A single-point Greek is a local derivative and can fail during jumps or volatility-surface repricing.
  • Distinguish share-equivalent, dollar Delta, and dollar Gamma per 1% units before comparing providers.
  • Treat zero-Gamma or “gamma flip” levels as model roots that move with time and inputs, not hard support or resistance.
  • Stress event gaps, expiration, market-on-close activity, liquidity withdrawal, and other participants whose flows may dominate dealer hedging.

Common misconceptions

Section titled “Common misconceptions”
  • “Open interest reveals dealer positions.” Every contract has a long and short side; OI does not identify either holder.
  • “Positive GEX guarantees low volatility.” It suggests a possible dampening hedge direction under assumptions, not the size, timing, or dominance of that flow.
  • “Negative GEX predicts a decline.” Short-Gamma hedging is procyclical in either direction; it concerns amplification, not a bearish forecast.
  • “A large strike is a price wall.” Concentrated exposure can move, be offset elsewhere, or matter less than news and underlying-market liquidity.
  • “All GEX dashboards should match.” Gamma models, signs, expirations, data snapshots, scaling, and units often differ.
  • “Dealer hedging is mechanical and immediate.” Dealers can net risks across books, tolerate bands, choose different hedge instruments, and rebalance discretely.
Section titled “Related topics”

Authoritative sources

Section titled “Authoritative sources”