CMYK support is getting there but very slowly.

You can import, export, and proof in CMYK today and you can have a CMYK color profile. GIMP 3.1.2 added total ink coverage for CMYK so 3.2 will have that. However, everything is still handled as sRGB internally.

With GIMP 3, all the required plumbing is there to handle CYMK natively. If I had to guess, I would expect it around GIMP 3.4. So, probably 5 years or more from now.