Gate officers were Imperial Navy Troopers stationed onboard Golan M3185 space stations in charge of operating its Shield Gate.
In 0 BBY, gate officers that were part of Gate Control operated the shield gate of the space station orbiting the Planet Scarif. Rebellion members operating under the callsign Rogue One and piloting a stolen cargo shuttle equipped with Imperial codes were able to trick the gate officers, who had been initially suspicious, into lowering the shield gate for them, allowing them to land.
A gate officer pressing a button to close the Scarif shield gate.
Later on, Rebel ships began dropping out of hyperspace and engaged the Imperial fleet orbiting the planet. Blue Squadron was tasked with getting to the planet's surface and began passing through the still open shield gate. Seeing this, the gate officers scrambled to close it. When they did so, they managed to destroy two X-wing starfighters who crashed into the energy field.
As the battle progressed, a Sphyrna-class Hammerhead corvette, Lightmaker pushed a disabled Imperial I-class Star Destroyer, Persecutor into another, the Intimidator, shredding the latter ship. Continuing to push, the Lightmaker, aided by the planet's gravity, sent itself, the disabled Persecutor, and the debris of the Intimidator into the Shield Gate, destroying it and lowering the planetary shield.
These Imperials first appeared in Rogue One: A Star Wars Story. They were identified as Gate officers in the movie's subtitles on Disney+.