My Photoshop course continues and in the classes where my homework was targeting the computer display my plan worked flawlessly: I used GIMP to do all the work and when done just saved as .PSD, I enjoyed the experience a lot. Now this strategy hit a roadblock: I have to create some stuff for print, as CMYK files in .PSD format. Total failure.
To be honest, I pretty much expected that, I know GIMP misses those features, I know they are listed in a dusted TO DO somewhere and I know at some point they may get implemented. To be totally honest, if I have to do such graphic things like book covers or posters, GIMP is not my first choice, I would prefer to use Inkscape, since I like better its workflow and thing vector graphics are better suited for the task, but it has the same CMYK problem, and I know the more appropriate job for the task is Scribus, it will create print-ready files.
But that is out of the scope, when I do my own stuff, I choose my own tools. For now I had a clearly defined homework: create a design as CMYK .PSD file. A good opportunity what work and what does not in GIMP.
And if you insist in editing a CMYK .PSD with GIMP, of course you can use Photoshop to convert CMYK to RGB, open with GIMP, edit, save as RGB .PSD, open in Photoshop and convert again in CMYK. Again, some color loss is possible.
On the bright side, the GIMP color picker has some usability features: a list with the recent used colors, the ability to sample a color from anywhere on the screen and multiple modes (I prefer the color circle).
Besides CMYK, there is another important part when working for print: instead of pixels, all your measurements will be in millimeters (or inches, if you are from USA).
Still, no way to produce print-ready files with GIMP?
Let me clarify: if you want to print a color image to your home or office printer and you can create only RGB files, there is no problem, it will be just fine, maybe no 100% accurate in colors, but close enough. You need CMYK only for professional, industrial printing (and maybe not even then, I sent photos in RGB JPEGs and they came out really well for some photo exhibitions). But if you need CMYK, there is a way.