Hi Antoine,
You didn't say what acquisition software that you are using, but different programs store different information. I use SGPro and the ASCOM driver for my ASI294MC-Pro camera, and the Bayer matrix pattern is not available through the ASCOM driver. If I use NINA, it uses the ZWO native camera driver and this does have access to the Bayer matrix type and records it in the FITS file header as far as I know. When you take your image in NINA, you can switch on automatic debayering and it should preview the image in colour after applying an unlinked stretch - you'll need to select these options in "Options\Imaging"
If the FITS file does not have a Bayer matrix type, then you will need to tell the Debayering software what the matrix array is. So, for example, in PixInsight, for any images that I take using SGPro (no Bayer matrix data), I have to tell the Debayer module that the matrix is "RGGB". In SIRIL, I believe that it automatically assumes that the pattern is RGGB unless the FITS header tells it otherwise. I don't use GIMP, but does it have a Debayer option - if not, it will display in B&W unless you Debayer the file first externally?
I've checked in ASI Studio's ASIFITSView, and it also seems to require the BAYERPAT keyword if it is to display files as colour images rather than monochrome. If you use ASIFITSView to open one of your images, you can right-click on it and it will give the option to view the FITS header and you should be able to see whether the BAYERPAT entry is there.
Terry
