The GEG reduces the damage and crew from every weapon attacking the ship by its rating (3 in this case). Except for Crits, which go straight through.
Example: an Omega scores 6 hits with its Double Damage beam.
Rolling for damage: 2, 2,3,4,5,6.
4 standard hits = 8 damage, 8 crew.
This is reduced by three: the ships suffers 5 damage & crew loss.
The crit goes straight through, doing 2 damage, 2 crew and the additiona effect is 1,1 (typical!) so this does 2 damage, 2 crew and -1 speed.
Total = 7 damage, 7 crew.
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Example 2: 4 Aurora starfuries surround the Adira.
Each rolls 2AD and so cannot damage the Adira unless they score a crit.
One gets lucky, 2 hits, but one crit. The normal hit will be completely ignored. The crit will go straight through.