My avionics dealer won't do an IFR cert if the encoder for the Garmin navigator or transponder is the GEA 12 because they always test the components and altimeter on the bench together. Since the GEA encoder stays behind with the equipment tray and is wired in they can't do the check outside the aircraft the way they like. There may be other more sophisticated shops who can do a cert on board the aircraft or know how to extract the wiring harness for a bench check (unlikely).
As a result I recommend a small Trans-Cal digital encoder for close to the same price as the GEA 24. All that is needed is a one-conductor shielded wire to send altitude to the navigator/transponder. And it can be tested on the bench in a conventional method.
Jim