I've heard the term Gold master came about from a misunderstanding,
the term Glass Master Disk or GDM was used for the master disk for a CD-Rom. People began using the term Gold Master Disk becuase writeable disks were gold and they presumed that these were used as the master CD from which the rest would be made.
While in fact the master disks are made of glass.
But for Software to reach "gold" it was to be the final complete retail version.