Contribution by strike
Each strike contributes its own slice of gamma exposure, depending on how many contracts are open there, their gamma, and the sign convention. Strikes near the price, and near expiry, contribute the most, because that is where gamma is largest.
Example: SPY at $500 has 50,000 contracts open at the $550 strike and 10,000 at $502. The $550 strike holds five times the contracts, but if its gamma is 0.002 and the $502 strike's gamma is 0.03, the $502 strike contributes 30,000 shares of delta change per $1 (10,000 × 0.03 × 100) and the $550 strike 10,000. The busiest strike is not the most sensitive one.
When levels cluster
Sometimes the call wall, the Gamma Flip Level, Max Pain and the strike with the most open interest all sit near one price. That makes the price worth watching.
Keep in mind that all four come from the same open-interest data. Clustering means they agree, but it is not four independent pieces of evidence.
Turning exposure into shares
Dividing GEX at a strike by the stock price gives shares of delta change per $1 move. That is a useful sense of scale: it estimates how much stock a fully hedged dealer would need to trade if the price moved $1 from here.
Caveat: it is a local estimate under the sign convention, not the amount dealers have traded or will trade. Expiry dates matter too: a strike with large exposure that expires tomorrow is gone the day after.