I see the separate+ plugin there

This is an old plugin, but all that is available for Gimp. Nothing even on the horizon for CMYK support in future versions.
Depends what you want to do. These days for large photographic prints, send your RGB image off and it is printed using either a large inkjet or a colour laser. If your printing company wants CMYK find another printer.
Might be a bit pricey but small numbers of posters can be done this way as well.
Large volume, posters, brochures, flyers, offset printed will (usually) need a CMYK image. (some printers will convert for you)
The important thing is ask the printer for their particular requirements, they might even supply a particular .icc profile or stipulate one.
Another bit of reading for you
http://www.northlight-images.co.uk/arti ... part2.htmland if you want some .icc profiles there is a package, it will be there in 'buntu as icc-profiles-free same as debian.
list of files included:
https://packages.debian.org/jessie/all/ ... e/filelistYou do not actually need separate+ there are on-line convertors;
http://www.rgb2cmyk.org/Remember Separate+ is a final step with your image. All editing is RGB.
For best results your work should start in CMYK colorspace. Which in linux leaves Krita as the option.