I believe he means the
the gpt.bin is the partition table. I suspect it sets the partitions for everything else that gets flashed. If someone else can give a good explanation for what it is, what it does and why it can't be downfgraded I'd love to hear it.
I've also read elsewhere that some people have gotten around the error by flashing just the gpt.bin in fastboot. Don't see why that would work though.
If gpt.bin is a partition table and it doesn't match up with all of the files 100% then an error would be generated, correct? Is this why you are saying it is unlikely that swapping out this file would work?