Definition - What does Golden Master (GM) mean?

GM stands for Gold Master. It's the finalized firmware before public release.

Golden master is a stage in the software release cycle in which the software is in its full and final development stage and is ready to be delivered to the customer or to be published commercially. The term was initially coined by Apple Computers to specify software products that have passed the entire production/development and testing phases.

Golden master is also known as the release to manufacture (RTM) version and the gold version.

Golden master is the final version of software development, succeeding alpha, beta or other software development versions. Typically, the golden master doesn’t require any further programming or testing and can be readily used. However, digital signing might be required to ensure the software's integrity and authenticity. This version is generally sent to the manufacturing department, where it is replicated into numerous copies. Besides Apple, most software companies and producers of software-enabled products use the term golden master or gone gold to indicate software that is ready to be delivered to end users.

Source: techopedia