What does GMK category mean in Karnataka KCET?

GMK = General Merit, Kannada-medium. It is reserved for General-Merit candidates who studied in Kannada as the medium of instruction for the qualifying years (as defined by KEA). If you are eligible, you compete in the GMK pool as well as the open GM pool, and take whichever closing rank gives you the seat. Confirm the exact eligibility conditions on the official KEA website.