CMYK color space is a field where GIMP is a bit behind. But on the site “Linux for Designers” of Eckhard M. Jäger you can find two plugins and information how to use them. They are called separate+ and cmyk-tiff-2-cmyk-pdf.
Just to fill the week between the episodes – some links for you. Find images under Creative Commons licenses with matching colours at TinEye. Creative Commons?