What officially gone gold means is that the game is now content complete and it is now prepared for shipping. In the meantime, the game developers will now be working out the kinks of the final product that might be present in the game. These problems will be fixed in a day one patch, which has been a common standard in the gaming industry these days.
Shadow of the Tomb Raider has gone gold! Lara’s defining moment is out of @EidosMontreal’s hard-working hands & on its way to manufacturing. Shadow of the Tomb Raider will be available for the Xbox One, PlayStation 4, and Windows PC/Steam on September 14. pic.twitter.com/6XTwzuMVnP