It’s the digital equivalent of searching for "cold fusion reactor DIY kit." The intense desire is real, but the simple solution does not exist.
GIMP 3.0 (recently released) still does not have native CMYK editing. The developer team has stated it remains a long-term goal, but it's not there yet. gimp cmyk plugin download
That's an interesting search query because it highlights a long-standing for many GIMP users. It’s the digital equivalent of searching for "cold