GIMP support soft proofing CMYK colors while editing your images, meaning you can see what your images would look like printed on paper or another printing medium. Since GIMP edits in RGB color spaces only, this is an effective way to edit your images in GIMP with printing in mind without actually editing in CMYK.
In order to soft proof CMYK colors, you need to download color profiles to your computer and tell GIMP where those profiles are located. Luckily, there are plenty of free color profiles out there, including for both CMYK and RGB color profiles.
Where to Download
You can download color profiles for free directly from the ICC (International Color Consortium) website here, or you can download Adobe’s color profiles directly from Adobe here.
Once you’ve downloaded the color profiles to a location on your computer, you can watch my tutorial on how to setup CMYK color profiles in GIMP for a step by step tutorial.
As of 2022, GIMP is making tons of progress with its development versions (starting with GIMP 2.99.12) towards adding more CMYK support and features. This progress means we should see some robust CMYK features in GIMP 3.0, and perhaps even full CMYK support (i.e. editing images in true CMYK color profiles) in GIMP 3.2.