From one option to the whole chain
Every option has a gamma: the amount its delta changes when the stock moves $1. Multiply that by the open interest at a strike and by 100 shares per contract and you get the change in delta, in shares, for everyone holding that strike. Multiply by the stock price and the answer is in dollars. Add up all strikes and you have gamma exposure.
Example: SPY is at $500. A strike has 10,000 call contracts open, each with a gamma of 0.02. That is 0.02 × 10,000 × 100 = 20,000 shares of delta change per $1 move, or about $10 million. If dealers own those calls, a $1 rise adds 20,000 shares of delta to their book, and to stay hedged they sell about 20,000 shares. A $1 fall makes them buy about the same amount.
Whose gamma? The sign convention
Orogex counts total open interest, every contract outstanding, because open interest does not say who owns each contract. Any GEX number therefore rests on an assumption about who holds what. Orogex uses a widely used one: dealers are long calls and short puts. Calls therefore count as positive GEX and puts as negative. Research supports the put side for index options, where end users are net buyers of puts (Gârleanu, Pedersen & Poteshman, 2009). The call side is a simplification, and for single stocks the evidence is weaker.
Under that assumption, positive net GEX reads as dealers being long gamma: they would sell as price rises and buy as it falls, which leans against the move. Negative net GEX reads as dealers being short gamma: they would buy as price rises and sell as it falls, which leans with the move. If the real dealer book differs from the assumption, so does the hedging. Research has linked hedging of this kind to intraday momentum and to how volatile prices are (Baltussen et al., 2021; Ni, Pearson, Poteshman & White, 2021).
The levels on the map
Orogex draws three landmarks from the net GEX at each strike, for the expirations you select. The call wall is the strike with the largest positive net GEX. The put wall is the strike with the most negative net GEX. The Gamma Flip Level (GFL) is where net GEX changes sign between two neighboring strikes, closest to today's price, estimated by drawing a straight line between them. The regime label is the sign of net GEX at today's price.
Caveat: GEX measures sensitivity, not orders. It shows how hedges would need to change if dealers hold the assumed positions and hedge fully. Dealers may hold different positions, offset risk with other options, or hedge late. And a GEX bar can change because prices and volatility moved, even if no contracts were added. The exact definitions are on the Methodology page.