Gimp does not have a native CMYK mode. There is a plug in that separates the image into CMYK layers, but then you can't adjust the compression options to save it as a TIF. I love working in GIMP, but this is one of the serious issues with it...
The people who are improving GIMP are trying to fix this, but it isn't on top of their to do list (unfortunately).
Basically, I still have to bring my image into Photoshop and change the mode there when I need to. It is really quite a pain.