That "header encoding" doesn't look like an encoding at all, as most of the bytes are zero and some header structure is retained.
It should be something simple. Try guessing it
Alternatively you can try to disassemble/debug Garmin for Android/Garmin for iOS/BaseCamp/MapSource. That apps shouldn't be too hard to understand, as they are not protected from reverse engineering and they should have real function names inside (except MapSource).
Alternatively you can try to find someone skilled and interested, who can do this for you. But I think very few people actually need it And everyone is lazy nowadays. Not like it used to be in the golden age of Garmin-o-hacking.