Both darktable and RawTherapee have an black level offset, compared to Exif. For RawTherapee it is Exif (128) + 16. darktable has it locked to 143. Increasing this to 144 or 145 makes the image less red.
If you want to calculate your black level GitHub - lightful/hraw: Hacker's toolkit for image sensor characterisation and a black raw file should work. See here for more about black level and hraw Measure black and white levels?
In the sample 1Ds, like your GX9, doesn’t have any hidden black fields to analyze. Therefore a picture with lens cap on should work.