The one who tends a threshold — deciding what passes by declared law, never by whim; a tending agent's seat in this house.
Compound formed in Modern English from gate (Old English geat) and keeper (from Old English cepan, to keep, guard); the one who tends a threshold, deciding what passes by declared law rather than by whim.
Passage through a threshold is decided by a rule written down beforehand rather than by mood in the moment, which is what separates tending a gate from merely standing in front of one.