Someone who makes or tends gates, or a dialect term for a gutter or watercourse.
From 'gate' plus the agent suffix '-er' (one who does something), or from 'gat' (an archaic word for a waterway or channel).
The '-er' suffix is so productive in English that almost any verb becomes a person—'gate' becomes 'gater,' 'teach' becomes 'teacher'—which is why we can instantly understand job titles we've never heard before.
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