GEX Explained Simply: Why Some Days the Index Sits Still and Others It Crashes
Hi trader,
let me tell you about two days, take the same index, put it in the exact same context.
Day one. The price wakes up, ticks up a hair, ticks down a hair, and for eight hours it stays glued to a level like a barnacle on a rock. Every attempt to break higher gets sold, every little dip gets bought back. Deadly boring.
Day two. Same index. It opens lower, and instead of finding a cushion it finds thin air. Every bounce lasts three minutes and gets wiped out. The price doesn’t fall, it plummets.
The question is: what changed?
Often it’s not the news. It’s who is forced to buy and sell behind the scenes, without anyone asking them to, just to do their job. That “who” is the market makers, and the compass that tells us which way they’re pulling is called GEX, Gamma Exposure.
It’s calculated on the options of an underlying, usually the US indices like the S&P 500 (you’ll find it as SPX), or the ETFs that track them, like SPY, which tracks the S&P 500, and QQQ, which tracks the Nasdaq-100. In practice it tells you how much gamma the market makers are carrying on that market, and therefore which regime you’re in.
Now let me explain it without a single formula, promise.
Let’s start with the market maker
When you buy or sell an option, on the other side there’s almost always a market maker. He’s the one running the house, always ready to give you a price, to buy or sell at any moment.
But careful, he doesn’t want to bet on direction. He couldn’t care less whether the market goes up or down. He makes his money on the spread, on making the market, not on guessing where it goes.
The problem is that the moment he sells you an option, he’s left holding a directional exposure he never wanted. So what does he do? He neutralizes it. He goes to the underlying, the future or the index, and buys or sells just enough to get back to flat.
It’s called delta hedging. Covering. Put even more simply: every time the market moves, the market maker has to adjust his hedge to stay neutral.
There you go. Hold on to that picture, because that’s the whole thing: a guy forced to buy and sell the underlying non-stop, not because he believes in it, but so he doesn’t get caught leaning.
Gamma, without the jargon
Delta is how much the option moves when the underlying moves. Gamma is the speed at which that delta changes as the price runs.
Translated into practice: gamma is how often and how hard the market maker is forced to readjust his hedge.
And here’s where it gets good, because that readjusting can do two opposite things. It can slow the market down or it can step on the gas. It depends on which side of gamma the market makers are on as a group.
Let me put it in racing terms, which are mine.
The market makers’ hedge is like your car’s suspension. When it works well, you hit a pothole and the car soaks it up, you stay glued to the asphalt. When it’s shot, that same pothole sends you flying, you bounce, you lose grip, and on an icy downhill you leave your skin on the road.
Same asphalt, same pothole. What changes is the suspension.
GEX is the way to know, before you set off, whether today the market has good shocks or blown ones.
Why they hedge, and how they really do it
Let’s take it in order, because this is the heart of everything.
When market makers as a whole have positive gamma, their hedging leads them to do this:
- the price goes up, they sell a bit of the underlying
- the price goes down, they buy a bit of the underlying
They buy the dips, they sell the rips. They go against the move. They’re the shocks that work.
When instead they have negative gamma, the exact opposite happens:
- the price goes up, they have to buy even more
- the price goes down, they have to sell even more
They chase the move, they pour fuel on it. Blown shocks.
Same job, same people, opposite effect on the market. That’s the whole trick.
Positive GEX: the market that stabilizes itself
When GEX is clearly positive, the market makers act as a brake.
Every time the price tries to break away, their hedging pulls it back. Result: compressed volatility, small moves, plenty of mean reversion. The price glued to a level.
That’s day one of my example. The barnacle on the rock.
In practice
You’re in a positive gamma regime. The future opens, tries to break above a high, makes ten points and fizzles. Comes back inside. Tries to break below, another ten points, and bounces. It keeps going like that until the close, inside a tight range.
If that day you go hunting for explosive breakouts, you’ll get ground down by a thousand false signals. The market isn’t messing with you, it’s just telling you that today the ones who brake are in charge.
Negative GEX: the market that amplifies
When GEX turns negative, the scene flips. The market makers go from brake to accelerator.
The price falls, they sell, the price falls even more. The avalanche kicks in. Here come the gaps that don’t fill, the trends that accelerate, those air pockets where the market seems to be walking down the stairs with no handrail.
That’s day two. The void under your feet.
In practice
Same index, but below a certain level you’re in negative gamma. A sell order comes in, and instead of finding buyers it finds more forced selling. A move that on a normal day would be twenty points becomes eighty.
Here the breakout isn’t a false signal, it’s the day. And if you start buying the dip “because it dropped too much”, you’re trying to catch a falling knife.
Same strategy, two days, two opposite outcomes. Not because you’re better or worse. Because the regime is different.
The three levels that matter
GEX isn’t a single number, it’s a map. And on this map there are three points worth knowing.
Zero gamma, or gamma flip. It’s the borderline. Above it, the market tends to stabilize (good shocks). Below it, it tends to amplify (blown shocks). It’s the most important level of all, because it tells you which world you’re in.
Call wall. It’s the strike loaded with calls above the price. In a positive gamma regime it works like a ceiling, a resistance that tends to slow rallies down. The market gets up under it and struggles to punch through.
Put wall. It’s the strike loaded with puts below the price. It tends to act as a floor, as support. But watch out: if the price breaks through it, that’s often exactly where you slide into negative gamma territory, where there’s air underneath.
Zero gamma, ceiling, floor. Three levels, and you’ve already got an idea of where the price breathes and where it risks running.
How you actually use it (read this part carefully)
Here I have to be blunt, because this is where people get it most wrong.
GEX is not an entry signal. It’s the regime context.
It doesn’t tell you “buy here” or “sell there”. It tells you what kind of day you’ve got in front of you, and therefore how you should behave. It’s the difference between driving on dry roads and driving on wet ones: it doesn’t change where you want to go, it changes how you get there.
In a positive gamma regime:
- expect range and mean reversion
- breakouts tend to fail, so distrust the breaks
- targets are shorter, the market won’t hand you the big move
In a negative gamma regime:
- expect moves to have follow-through
- trend and momentum work, “it dropped too much” doesn’t
- volatility is higher, so keep your size smaller and your stop wider, otherwise the noise shakes you out
See what changes? Not the entry. What changes is size, stop and type of strategy. That is, the three things that decide whether you survive.
And here’s the uncomfortable truth. This map, the gamma flip, the call wall, the put wall, a lot of people don’t have it in front of them. They trade in the dark, then they’re surprised when the breakout fails on them five times in a row.
Working it out by hand from the open interest is a brutal pain. That’s why we put it inside Thunder Desk: gamma exposure on the S&P and Nasdaq, updated, free, without you having to download an options spreadsheet and pray. You open it, you look at which regime you’re in, and you already know whether today you go hunting for range or for trend. No noise, just data.
This isn’t a banner I’m selling you. It’s the difference between knowing it’s going to rain before you head out, or noticing when you’re already soaked.
The limits (because it’s not a crystal ball)
Now the cold shower, which is only fair.
GEX is an estimate. It’s built on the open interest of the options, that is, on how many contracts are open at each strike. But to turn that into “the market makers are long or short gamma” you need an assumption about who bought and who sold each piece.
And that assumption can be wrong. It’s not always the case that whoever bought that call is a client and whoever sold it is a market maker. The models make a reasonable average, but it’s an average, not an X-ray.
On top of that, the picture changes after OPEX, the monthly options expiration. When that mountain of contracts expires, the gamma resets and rebuilds from scratch. A level that held beautifully on Thursday can evaporate the Monday after expiration.
So treat it for what it is: a compass on the regime, not a GPS that drives you into the parking spot.
Three things to take home
- GEX tells you whether the market makers today are braking the market (positive gamma, range) or amplifying it (negative gamma, trend and sharp moves). It’s the regime, not the signal.
- Three levels are enough to orient yourself: zero gamma (the border), call wall (the ceiling), put wall (the floor).
- It doesn’t change where you enter, it changes how: size, stop and type of strategy. And it’s an estimate, not an absolute truth, so watch out for OPEX.
And here I’ll leave you a hook for the next piece, because there’s something that in recent years has made this whole picture a lot more jittery.
Until not long ago you calculated gamma on the weekly and monthly expirations, fairly stable stuff. Then the options that expire same-day arrived, the 0DTE, and they sent intraday gamma on a roller coaster: it builds up and resets within the same session, and the map you had in the morning is already a different one by noon.
We’ll talk about them in the second article of the series, entirely dedicated to them:
Coming Friday
The second piece in the series: 0DTE, the options that expire same-day. How they work, why they sent intraday gamma on a roller coaster and what to watch during the session.
One last thing, just so we understand each other. The Events on The Thunder Trader run on forex, CFDs and crypto, in a demo environment, with no real capital at risk. GEX, on the other hand, is born on the index options, on the future. They look like two distant worlds, and yet knowing how to read the regime of the underlying is a competitive edge you carry with you everywhere, because when the future goes into negative gamma the rest of the risk doesn’t exactly sit still. It all moves together.
If you want to start from the basics in an orderly way, in the free ebook Win the Arena you’ll find the method step by step. Then put the Desk in front of you, and you never enter in the dark again.
I’ll wrap it up here. If you know someone who complains that “the market some days has gone crazy”, send them this article. Maybe it hasn’t gone crazy, maybe it just changed its suspension.
Suerte Amigo!
Tiziano Brunno
Tradingblog
Disclaimer: purely informational and educational content. It does not constitute financial advice nor an invitation to trade. Trading carries the risk of capital loss.
