I tried to inspect a bit more the situation:

rawtherapee which uses some custom dcraw success in extracting the compressed raw data,
but it fails in decoding the decompressed structure.
I modified it to dump the uncompressed raw (which display as garbage currently).

For a 25 mb pixels image, there is exactly 50 mbytes of decoded data being decoded.
which correspond to 2 bytes per pixel, which seems to be confirming the 16 bits per pixel.

I tried making a black frame and a white frame,
just to see if I could understand something, but the result are not that great:

The first bytes of the frames are looking like that:

white frame:

00000000: dfce f87f 7b8c ffdb d77c 9d99 7c14 29bd  ....{....|..|.).
00000010: 91a7 d027 0405 22d3 147d 3371 f16b 0b07  ...'.."..}3q.k..

black frame:

00000000: 1ba4 1b9c 1b80 1bb0 b707 f8d9 cdf8 12fa  ................
00000010: f71f 4e32 e6ad f321 9dff d53f 2731 ead1  ..N2...!...?'1..

So...
either they are crypted or salted,
either the colorspace is uncommon,
either I am missing something, which can be possible as I am far from an expert in this domain.

The data are there : (including the two RW2 files)

http://esby.free.fr/perso/GH6-RW2/

See
http://esby.free.fr/perso/GH6-RW2/P1000404-black.info
and
http://esby.free.fr/perso/GH6-RW2/P1000407-white.info

for the filesizes and basic descriptions

(the dng were producted by Adobe Dngconverter 14_3 under wine staging 7.2 )