How to convert image to CMYK in GIMP?

I realize this is an old question, but I just stumbled upon it and found the answer. After you separate the image into the CMYK channels, you get a .tif image with 4 layers (one for each color). This image looks inverted because each area where this is white/grey represents a certain amount of cyan, magenta, yellow, or black.

Once you have this, just go to Image -> Separate -> Export and export your image as a .tif. This will repackage your image with the CMYK profile you choose, and when you view it, it will be the right print colors.

You can also use Image -> Separate -> Proof, and select the color profile of your monitor and it will generate a .tif preview with all the right colors. It's a pretty nice plugin!

Super easy with ImageMagick (preinstalled in every Ubuntu):

convert input.jpg -colorspace cmyk -compress LZW output.tif

Note: the exported file would be quite large (no matter the tool), so it's better to include compression - for TIFF the most widely used is LZW.

More info e.g. here.

Have you tried this:

Separating an image:

To convert an RGB image to CMYK format, bring up the right-button menu, and go to "Image->" If the plugin in installed correctly, there will be a new menu, "Separate". From this new menu, select "Separate (normal)"; you will be prompted to select an RGB source profile, and a CMYK destination profile. If you have installed the Adobe and sRGB profiles as per the instructions in the archive, you can just accept the defaults for testing, otherwise you'll have to locate the profiles manually.

A new image will be created with four greyscale layers, named "C", "M", "Y", and "K".

If you have loads of memory to spare, you can use the "Separate (colour)" option; this will perform the same operation, but the new image will contain five layers: The first, "Background" will be white, and the other four will be solid Cyan, Magenta, Yellow and Black, with the separated image data in layer masks. In addition, the layer modes for the four colours will be set to "Darken Only". This gives a rough reconstruction of the colours, and is the next best thing to a true CMYK painting mode, since you can paint on the layer masks, and see the results in realtime.

NEW for 0.3 - the "primary" colours chosen for the "Separate (colour)" mode are now much more akin to the primaries used in printing, which are nowhere near as bright, saturated and downright lurid as pure screen Cyan and Magenta! This gives a far more realistic preview of the colours.

source : here

The Separate+ plugin for GIMP:

Load your file into GIMP. Make sure to save the file separately as flattened. Make sure you have registered any color profile you want to use (icc). If you have an ICC file you want to use; before you startup GIMP, right-click on it and the Windows XP menu will say "Install Profile" ... click that.

Back to GIMP... Click on Image > Separate Click on Separate in the menu... In the first dropdown click on your source (sRGB) more than likely. In the second dropdown click on the ICC that you want to use or provide the path. Click on Rendering Intent to select "relative colorimetric" Then just Click OK. You will get four layers, C, M, Y and K You can see it all put back together by Clicking Image > Separate > Separate then Proof If satisfied, you are ready to go. Click on Image > Separate > Separate and then Export. This you can use to combine all to CMYK format and save as a .tif. That is it in a nutshell with some variances... Hope this helps.

I gave up on Gimp as far as converting to cmyk - just install Krita its also open source and free. I no longer require photoshop. I use Affinity Publisher and will probably purchase Affinity Photo as well but for what I require now Krita works just fine. I simply added the cmyk profile I need, which is so simple to do in Krita.