Definition
When dealers are net long gamma, their hedging tends to dampen moves: they sell rallies and buy dips. When they are net short gamma, hedging amplifies moves: they buy into rises and sell into falls. GEX is calculated from open interest and assumptions about who holds the options.
Watch out for
Dealer positions are not reported, so GEX is a model estimate. Use it as context for how volatile conditions might be, not as a precise signal.