I see no answers, so I'm going to make up a few options...
1.) Gating factor was created to capture the feel of a horse race (on a track) where everything is ready to go and excited to go, chomping at the bit, but the gate is holding back the horses before the race starts. This captures the state of a project as ready to sprint - completely ready to proceed - except for the signal to open the gates.
2.) Gating factor was first used because the Panama Canal seemed to be a good metaphor. Projects and progress are ships, but in order to proceed to the end of their journey, they must occasionally enter a lock and it's attendant gate and assess forward progress. This can take some time while the lock fills with water. Sometimes ships sink.
3.) Gating factor - one day a manager looked upon his projects and lo, they were good. He imagined them as beautiful fields of grass and wild flowers, plowed under and replaced by neat, orderly rows of domesticated crops and set apart by ripping down native growth trees and replacing their sloppy branchiness with neat, structured, rock walls. The only way into the project to continue progress and eat of the crops is to stop at the gate and run one's security badge under the reader.
Title: Me? Was I promoted to middle management?
Snrky: We've identified you as the gating factor.