I'm pretty sure that due to the way Bungie created the code for Swordbearers and Gatekeepers. One Gatekeeper must spawn when ever a Swordbearer is killed. They're simply put up there behind the invisible wall so they don't run amok and completely ruin a teams chance at beating Crota.
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You are technically correct but in code during crota fight they can just put gatekeeper spawn to false when a swordbearer is killed. And it's not 1 per kill in there, there's always 2 active.