Looking at the .gcz files in the hex editor, it's clear that there is no header in the .gcz files, at least in the first two or three bytes. The headers in all of the .gcz files are wildly varied but "GC" is always at the fifth byte. I'll provide an example header just to give you an idea of what I'm talking about:
Attachment:
File comment: An example of what a PPP .gcz header might look like.
gcz bg header.png [23.09 KiB]
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EDIT: In case you can't see the picture, the first few bytes are highlighted in the hex editor to show what the "header" (or lack thereof) looks like.