Understanding Gamma Exposure (GEX) in 0DTE Options

What is Gamma Exposure (GEX)?

Gamma Exposure (GEX) refers to the sensitivity of option market makers (dealers) to changes in the price of the underlying asset.

When retail and institutional investors buy or sell millions of 0DTE options, the dealers on the other side of those trades do not want to take directional risk. They want to collect the bid-ask spread. To stay delta-neutral, dealers must constantly buy and sell the underlying asset (e.g., S&P 500 futures or SPY shares) as the market moves.

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<text x="400" y="340" font-family="Inter, sans-serif" font-size="14" font-weight="bold" fill="#334155" text-anchor="middle">Underlying Price (Relative to Strike)</text>
<text x="60" y="175" font-family="Inter, sans-serif" font-size="14" font-weight="bold" fill="#334155" text-anchor="middle" transform="rotate(-90 60 175)">Gamma Sensitivity</text>

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<text x="400" y="40" font-family="Inter, sans-serif" font-size="14" font-weight="bold" fill="#0f172a" text-anchor="middle">Strike Price (ATM)</text>

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<text x="650" y="270" font-family="Inter, sans-serif" font-size="12" font-weight="bold" fill="#64748b">30 Days to Expiration</text>

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<text x="650" y="250" font-family="Inter, sans-serif" font-size="12" font-weight="bold" fill="#2563eb">7 Days to Expiration</text>

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<text x="450" y="100" font-family="Inter, sans-serif" font-size="14" font-weight="bold" fill="#1e3a8a">0DTE (Expiration Day)</text>

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<text x="415" y="130" font-family="Inter, sans-serif" font-size="12" fill="#334155">Maximum Gamma at Strike</text>

Positive vs. Negative Gamma

The aggregate positioning of these dealers creates either a Positive Gamma or Negative Gamma environment, which drastically alters how the 0DTE market behaves.

Positive Gamma Environment

When the market is in a positive gamma regime, dealers are long gamma.

• The Mechanic: As the market drops, dealers buy the underlying. As the market rises, dealers sell the underlying.
• The Result: Dealers act as a dampening force. Volatility is suppressed, and the market tends to chop sideways in a tight range. Mean-reversion strategies (buying the dip, selling the rip) work exceptionally well.

Negative Gamma Environment

When the market is in a negative gamma regime, dealers are short gamma.

• The Mechanic: As the market drops, dealers must sell the underlying to hedge. As the market rises, dealers must buy the underlying to hedge.
• The Result: Dealers act as an accelerating force. Selling begets more selling, and buying begets more buying. Volatility explodes, resulting in large, unidirectional trend days.

The Zero Gamma Level (Vol Trigger)

The price level at which dealer positioning flips from positive to negative is often called the “Zero Gamma” level or “Vol Trigger.” For 0DTE traders, this level acts as a critical pivot point. If the SPX breaks below the Zero Gamma level intraday, 0DTE options pricing will rapidly expand as dealers are forced into directional hedging, accelerating the market’s descent.

0DTE Volume and GEX

Because 0DTE options account for roughly half of all SPX options volume, intraday 0DTE trading can actually change the GEX profile of the market in real-time. A sudden influx of 0DTE put buying can force dealers into a negative gamma posture, sparking an intraday selloff entirely driven by options mechanics rather than fundamental news.