Shield gating occurs when a Player or their companion’s shields reach zero. All excess damage done is negated and they becomes immune to damage for 1.3 seconds.
Enemies only gain 0.1 seconds of 95% damage reduction when their shield gate occurs. Hitting an enemy’s weakpoint will bypass shield gating entirely, allowing hard hitting headshots to still one-shot enemies through shields.
Shield gating prior to fully recharging shields will result in only a 0.33 second immunity; to get the longer immunity back, shields need to FULLY recharge. Low max shields and fast shield regeneration can help exploit the immunity aspect of shield gating.
For more, check out the wiki page on Shields.