Net GEX is usually quoted as a single figure for the whole chain. On an index with daily expirations that number is a blend of two very different things: the contracts that stop existing tonight, and the contracts that will still be hedged tomorrow morning.
Why the blend misleads at the close
At 15:00 the 0DTE slice dominates the total — it has the largest gamma because it is closest to expiry. It is also the slice about to vanish. A desk reading only the blended number sees a strong regime, walks into the close expecting it to hold, and arrives the next morning on a completely different map.
What the per-expiry view shows
- Which expiration actually carries the book — sometimes it is not today.
- Whether the sign flips once tonight contracts are removed.
- How far out the exposure runs, which tells you whether tomorrow starts from structure or from scratch.
The case that changes your afternoon
A session where the 0DTE slice is strongly positive and the next expiration is negative is not a positive-gamma session. It is a positive-gamma afternoon followed by a negative-gamma open. The pin you are trading into is real, and it has a deadline.
The reverse case is more comfortable and just as useful: when the sign agrees across expirations, the regime is not a same-day artefact, and a level that held today has a reason to matter tomorrow.
How to use it without overreading
Read it twice a day. Once around midday, to know what kind of afternoon the book is set up for; once near 15:00, to know whether the level you are leaning on expires with the session. That is the whole workflow, and it takes about ten seconds each time.