Postal Dude enters the complex in his search for the gold master until he arrives to Bullfish Interactive CEO Phraud Hogslop's office. The Dude tells Hogslop that he has come to pick the gold master up, but Hogslop refuses to give it back. The Dude picks it from Hogslop's desk and tries to go away with it, but is confronted by Hogslop in a boss battle. After killing him, the Dude manages to take the gold master to a party with the Postal Babes at Vince's House.
The Gold Master itself is not a tangible item, and cannot be picked up or added into the inventory. It's just a visual prop.