Okay ... here we go ...
RGB is a transmissive and additive color model while CMYK is reflective and 'subtractive'. What this means is for RGB you take varying amounts of colored light and add them together to create colors based on the ammount of Red, Green, or Blue light you add. Depending on the number of bits in the signal controlling the light source you can have 24, 64, 256, etc. variation in intensity. The most common is 32 bit color where there are 24 bits (one byte per color) plus 8 bits for an alpha channel to determine transparency. The give you 256 options per channel. An RGB value of 0, 0, 0 would be black since there is no light being transmitted. An RGB value of 255, 255, 255 would be white since all colors are on at 100%. The eye gets the full visible spectrum.
Now a reflective color model like CMYK requires a light source which can change the color the eye sees based on it's own color temperature. The ink that is put down absorbs certain colors of light and reflects others. Cyan ink, for example reflects blue, but absorbs everything else. To simplify things (and be somewhat incorrect in the process
) the black channel is for contrast and detail. Theoretically you should be able to achieve black by mixing equal amounts of cyan, yellow and magenta ink, but because of the pigments and binders, it never gets to 100% so black is necessary.
In any case, when you see a color image on an opaque surface, it is most likely printed in CMYK. There are other models used for printing but far and away CMYK is the most prevalent. One thing you can be certain of, though, is it will NEVER be RGB.
What's going on here is that technology is getting better into making images consistent in different mediums. A truly WYSIWYG interface is simply making those visual adjustments to remove those colors that are not available to both color models. There are colors that RGB can display that CMYK cannot reproduce, and vice versa. WYSIWYG doesn't allow those colors to be used so the translation tables to go from one to the other can be relatively accurate. You do indeed get your RGB images printed accurately, but they are printed in CMYK.
What you also have is a limited palette. While this is usually okay for a majority of work, professionals, especially those closer to pre-press production, need control over the CMYK channels and GIMP can't do that.
I've used both GIMP and Photoshop since the mid '90s. Both have come a long way and are both very powerful. Depending on your need GIMP certainly can be a good alternative to PS, but
@GameTheory 's point was specifically about commercial printing. I have to agree.
