Galatine is the sister-sword of Excalibur given to Gawain by the Lady of Lake. Referred to as the "shadow to Excalibur's light", the sword was hidden away in the shadow of Excalibur, so there aren't many mentions of it.

Then Sir Gawaine was all abashed, and with Galatine his good sword he smote through shield and thick hauberk made of thick mails, and all to-rushed and break the precious stones, and made him a large wound, that men might see both liver and lung.

Galatine is also the name of one of the domains in Kingdoms of Camelot.