Hmm. On my system (running Linux Mint), I get an error if I try to open an RW2 (don't have a CR3 file handy) indicating that darktable or rawtherapee needs to be installed. If I install darktable from the package manager, it just opens darktable when I try to open the file, and after closing darktable, GIMP loads the image.
How did you install GIMP and darktable? Are they both from the system package manager?
Worst case, if you're already using darktable, I guess you could just export the images after you develop the raws and then load them into GIMP from XCF/JPEG/TIFF/whatever files instead.