The dev version of #GIMP now supports importing Adobe Color Book (.acb) palettes! This is how Adobe stores the various Pantone, Trumatch, Toyo, HKS, and other print color palettes.
The files can't be included in GIMP directly, but now if you have them you can load them in for use with the other CMYK features.
In the future I'd like to add support for CxF as well as some FOSS palette formats like .kpl, .soc, and Scribus .xml.