What happens to gamma exposure (GEX) after monthly options expiration?

After a monthly options expiration, the contracts that expired simply stop existing, so their gamma leaves every gamma exposure (GEX) estimate at once. Whatever GEX picture you look at afterwards is rebuilt only from the contracts still open, which sit in later expiries such as the next monthly. This post walks through that mechanism on the Friday 2026-10-16 expiration, using three real SPY contracts that expire on Friday 2026-11-20.

What gamma and gamma exposure are

Gamma is how much an option's delta is expected to change for a $1 move in the underlying. The Options Industry Council notes that gamma is higher for options that are at-the-money and closer to expiration, and that with the stock right at a strike at expiration, an option's gamma is at its highest.¹

Gamma exposure adds that up across the options chain: model gamma per contract, times 100 shares, times the open interest at each strike. GEX is an estimate, not a published statistic, because it assumes which side of each trade dealers hold. Our post on what gamma exposure is and how to read it covers the formula and its assumptions in full.

Monthly expiration, and why it matters for GEX

Standard monthly options expire on the third Friday of the expiration month, or on the preceding business day if the exchange is closed that Friday, according to Cboe's SPX specifications.² That day is often called monthly OPEX. Our explainer on when options expire covers the rule in detail.

When a contract expires, or is exercised and assigned, it is closed and removed from open interest.³ Open interest is one of the inputs to every GEX estimate, so a contract that is gone carries no gamma into it. Any GEX estimate computed after the 10-16 expiration simply has no October-monthly contracts in it.

That is the whole mechanism. Nothing has to be traded for those contracts to leave the picture. The calendar removes them, and the gamma map is redrawn from what is left.

When: the October and November monthlies

October 2026 had Fridays on 10-02, 10-09, 10-16, 10-23 and 10-30. The third one, Friday 2026-10-16, was the standard monthly expiration, and those contracts have now expired.²

November 2026 has Fridays on 11-06, 11-13, 11-20 and 11-27, so the next standard monthly is Friday 2026-11-20.

Worked example: where SPY open interest sits after the October contracts

As of the 2026-10-16 session, CaymanBot's public delayed feed shows these three prints in SPY contracts expiring 2026-11-20.⁴ Each line carries the open interest the feed held for that contract, a once-a-day figure that does not change during the session. The gamma column is CaymanBot's per-contract model estimate, not an exchange figure.

These are not October contracts, so they were untouched by the 10-16 expiration. Lines like these are what a GEX estimate is built from once the October monthly is gone.

Turning one line into a gamma sensitivity

One contract covers 100 shares. Multiplying model gamma by 100 and by the open interest gives the change in share delta for a $1 move in SPY at that strike. For the $750 put line:

So, by the model, the delta of all open $750 November puts together changes by about 63,855 shares for each $1 move in SPY. That figure is a per-$1 sensitivity for one strike, computed from a model gamma. It is one input, and it is not a total GEX number for SPY; a full estimate repeats this for every strike and expiry and then applies the dealer-side assumption.

What the readings show

Gamma is higher for options near the money and near expiration.¹ The $750 put sat a little below SPY's price at the time of its print, and it carries the largest model gamma of the three. The $700 put and the $810 call sit further from SPY's price, and their model gammas are smaller. All three expire on 2026-11-20, so on 10-16 none of them had the near-expiry gamma that the October contracts carried in their final days.

How to read GEX after OPEX

Three habits help when you look at a gamma estimate in the days after a monthly expiration:

1. Check what the estimate is built from. After 10-16, it holds only contracts expiring later. A level that came from October open interest is no longer part of the picture.
2. Read gamma by strike, not just as one total. Large open interest at a strike near the stock price carries more gamma than the same open interest far away, as the three lines above show.
3. Remember the assumption. Every GEX number assumes which side dealers hold. Treat it as a model of hedging pressure, not a measured fact.

Expiration days are also when gamma at strikes right at the stock price peaks, which is why traders talk about pinning around OPEX. This describes the term only and makes no forecast for any date.

For SPY's own strike-by-strike gamma estimate, our SPY gamma exposure page is free, delayed at least two days and needs no login. To see the current strike-by-strike estimate across symbols, use the GEX heatmap.

How to practice this

Open a free CaymanBot account and look up SPY's gamma estimate on a day after a monthly expiration. Pick a strike with large open interest, multiply its model gamma by 100 and by its open interest, and check that you land on the share figure. In the first two Learning Mode stages, each trade on the feed comes with a plain-English sentence you can check against the strike, expiry and premium. The $100K paper account lets you place a trade in a contract you have studied with no real money at stake. A free account shows options data delayed 16 minutes (60 minutes without an account).

Educational only, not investment advice.