A gold master is a release candidate milestone which passes all of a publisher and platform's requirements. It is considered the finished game, locked and ready to be reproduced and sold.
Gold masters are not necessarily bug free, but bugs that they contain are generally not considered serious enough to block production. In older days when it was difficult to issue software patches for games (because of a lack of connectivity), publishers were often much less tolerant of bugs than they are today.
In online games the term is less widely used as those games have often already been released in the form of open betas to players.