GIMP doesn’t support CMYK yet. I think they were some plugins, but I am not sure if they work with the latests versions.
I think Sean’s approach is somewhat hackish and normally one could do well enough with a calibrated monitor, using tools that doesn’t destroy colors (for example, before filmic, I always got too saturated reds using the base curve, that destroyed skin tones) and trusting your eyes (in Sean case I think he developed that method because of his color blindness).