One of the things that graphic designers find necessary but, annoyingly, isn’t easy if you aren’t using Adobe’s Creative Suite is the ability to convert RGB to CMYK with GIMP as well as other proprietary pallettes such as the infamous Pantone. Because of licence restrictions on these sets of colours, they are not distributed with open source software so must be downloaded separately after the end-user agrees to their licencing terms.
Adobe RGB/CMYK Colour Profiles
To add Adobe’s colour profiles to GIMP, extract them into your home directory or somewhere you don’t mind having them sitting long-term, then; in GIMP, under Edit > Preferences, click the Colour Management section then select the profiles you’d like to use. A number of profiles are included in the bundle but most users will probably find themselves requiring the following:
Convert RGB to CMYK with GIMP
sudo apt-get install gimp-plugin-registry
Once installed, open GIMP and among other plugins that were added, there is now a Separate option at the bottom of the Image menu in the menubar.