This is not immediately relevant in Europe where ADS-B is not yet a factor, but is “interesting” in the way Garmin have handled it.
The new firmware needs a Field Approval. It isn’t covered by an STC.
The Field Approval route, Form 337, is used for a Major Alt when there is none of (a) TC (b) STC (c) a DER 8110.
It is the only time on a Major Alt when the FAA is involved in approving the modification. and this is now almost impossible to do for any aircraft based outside the USA, especially now that the Heathrow and Frankfurt FSDOs have closed.
The only way you will get it is via a contact in the USA willing to present your papers to an inspector. The question of where the aircraft lives is not really relevant and with a bit of luck will never be asked. I have done that twice; once for the Sandel SN3500 and once for the KLN95 GPS approach approval.
