I think there’s a behaviour vs implementation confusion - although the gearbox main axle isn’t implemented as a primary facing, setting it feels like setting the facing of many other GT blocks; although the gears aren’t implemented as covers, placing them feels a lot like placing covers on other blocks. Remember that the player can’t see the Matrix while they’re playing.
Anyway, as it happens I’m very glad that the gears aren’t covers, as that would have broken my first non-trivial axle/gearbox build: a silicon purification plant that runs on a single turbine. (Well, right this minute it runs on a single electric motor for testing, but next job is to replace that with a BC-fuel-powered turbine - the gas burners are so space efficient that I have an excess of unused BC fuel).
I have a chain of 5 mixers, each on a dedicated gearbox with an activity detector/redstone switch cover pairing, coupled to another gearbox on the main shaft which has the main axles turned perpendicular. When a machine has nothing to do its own gearbox shuts off meaning the coupled gearbox on the main shaft passes the RU down the line. (Apologies that the gearboxes aren’t visible - they’re there!)
One final embellishment to be added once the motor is swapped for a turbine is an extra gearbox at the end of the line. An “Activity Detector (Running)” placed on that final gearbox will only fire off a redstone signal when none of the earlier machines are drawing power off the shaft - coupled with a timer to filter out momentary stalls, that will be the signal to shut off the burning box. It’s a damn good job the gears aren’t covers after all…