The zero bytes of the GCD file is the clue indeed as it should have been ~20MB and was probably what was causing the bootloop. The device would have been recognising it as a firmware update file and so endlessly and fruitlessly attempting to process it to re-flash appropriate memory regions.

You may find that you'll need to load the aviation basemap manually as normally Garmin's programs, GarminExpress and WebUpdater, might not offer to replace it. If you need one and can't source it from a reliable backup or elsewhere let me know.

Anyway good result. I'll mark the thread as 'solved'. Safe flying.