The kimchi premium is the price difference between South Korean exchanges and other global exchanges for bitcoin. The price of bitcoin on a South Korean exchange may be higher than on the US or European platform. Some investors would attempt to benefit from the kimchi premium arbitrage by trading the price differences that occur across various exchanges.
gm is the short form writing for good morning. Used extensively within the crypto community where individuals greet each other casually. On the other hand, good night (gn) is used when people sign off for the day.