Long gamma
A position whose delta rises as the price rises and falls as it falls. Dealers who are long gamma hedge by selling rallies and buying dips.
Holding options, calls or puts, makes a position long gamma: each $1 move in the underlying pushes its delta in the direction of the move. To stay neutral, the holder trades against the move, selling shares after a rise and buying after a fall.
When a GEX model estimates that dealers are net long gamma, their combined hedging leans against price moves, which tends to damp them. This is why a positive gamma regime is associated with quieter, more range-bound trading. It is a tendency, not a rule, and it depends on the model's assumptions about positioning.
Illustrative example (hypothetical numbers, not a real trade): a dealer is long 50 calls with a gamma of 0.04 and is delta neutral. A $2 rise adds 50 × 0.04 × 2 × 100 = 400 shares of delta, so the dealer sells 400 shares. A $2 fall removes the same amount, so the dealer buys 400 shares.