I can neither answer on behalf of EASA nor Garmin. I have no relation to Garmin and none other to EASA than being a happy customer also helping others. I can only try to comment on your questions from my experience.
Originally Posted by
cessnapete
If a set of new avionics has been flying with proven safety for more than two years, in scores of different types of aircraft in the USA, why do EASA deem it unsafe to fit and fly in Europe?
I doubt your assumption is right. EASA does not deem these things unsafe, or did you hear of anything they pulled from the flying fleet? This kind of questioning will not give an answer, as it is not open for discussion. It is totally understandable you get no answer on such. If you want answers to the GFC500, you have to make the questions explicit, so they are answerable. I guesstimate the answer must and will be: ask Garmin, balls in their play-field.
Originally Posted by
cessnapete
And why does it take two years to check the manufacturers proven paperwork?
It does not. My experience is a receipt on next business day or day after and process finished within two business weeks.