The most fundamental shortcoming of the GIMP, according to graphics professionals, remains its limitation to grayscale and RGB image modes; press-ready images need CMYK (Cyan-Magenta-Yellow-Black), and many designers make heavy use of Lab color and duotone (tinted) modes. Second is its limitation to 8-bit color -- as high-end scanner and digital camera prices drop, more and more people need to work with 16-bit-per-channel data.
— Nathan Willis
AccordingFundamentalGraphics