It's been years since I last attempted to work with GIMP. When last I went to use it, it did not have raw conversion capability, could not do anything more than an 8bit per component image, and had no notion of color spaces or color-managed-printing. I needed four or five other applications to make using GIMP useful. Never mind that the installation and configuration of GIMP was a multiple hour learning endeavor before you even started to work on the application itself.
For me, that was too limited and inefficient a solution to be worth putting the learning and time investment into.
I hope that subsequent versions of GIMP have improved on this situation, even though I doubt I'm going in that direction any time soon.
G