What is the difference between GM, GMK and GMR in KCET?

All three are General Merit (open category), but for different pools: GM is the general/open pool, GMK is for Kannada-medium candidates, and GMR for rural candidates. If you qualify for GMK or GMR you are considered in that pool in addition to GM — which can help, because those pools sometimes close at a lower (easier) rank. Check all pools you are eligible for.