The calculation takes each strike's open-interest, multiplies by that option's gamma and by the contract size, and assigns a sign based on who is assumed to be long or short. The result is a dollar figure describing how many shares dealers must buy or sell for a one-point move.
The number is a model output, not a measurement. Nobody outside the clearing house knows which side of each contract the dealers are on, so every public GEX chart rests on an assumption — usually that customers buy puts and sell calls. Treat it as a hypothesis about flow, not a fact.
Example: a vendor reports $500 million of positive gamma at the $50 strike in XYZ. The implication is that a rally forces dealers to sell and a dip forces them to buy, damping movement near $50 until that open-interest expires or rolls.
Related: dealer-gamma, gamma-squeeze, max-pain, opex-effects