try separate +

It is a shame that no one has answered you on your CMYK queries but it is a big subject.

The CMYK_tiff-2-PDF plugin does what it says, can be a batch process, the file(s) does not have to be open in Gimp. Converts a CMYK tiff to a CMYK PDF. Tried it, it does work.

Gimp works in RGB mode. Commercial printers usually want CMYK files, these fit in with their printing process. Gimp will open a CMYK image file but converts to RGB. To save a RGB as CMYK you need to look at the 'separate+' plugins http://registry.gimp.org/node/471
There are binaries for MS Windows, if using a Debian linux there are binaries in the gimp-registry-plugins package.

Nothing is ever straight forward, you also need to know about color profiles, look it up on google or wikipedia, also some free files here
http://www.adobe.com/support/downloads/detail.jsp?ftpID=3680

There are other applications that do the RGB to CMYK conversion, usually Imagemagick on command line but recently Inkscape has some useful (beta) extensions.