Formally "Segragated Witness (Consensus Layer)" or BIP141, Segwit removes digital signature data and moving it to the end of a transaction to free up capacity. Transactions are essentially split (or 'segregated'), into two segments: the original data segment and the signature (or 'witness') segment. The declared purpose of Segwit was to prevent nonintentional bitcoin transaction malleability, allow optional data transmission, and to bypass certain protocol restrictions (such as the block size limit)
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.