Despite the trending idea that GDDs are obsolete and a thing of the past, they are still very much needed. Call it whatever you like (Wiki, GDD, Project Board, Bible…), it doesn’t change the fact that if you start the production without one, success will become highly unlikely for your game. GDD helps everyone on the team to stay on the same track and collaborate efficiently. Onboarding new team members becomes a piece of the pie with a first-class Game Design Document. It will also allow you to set accurate deadlines and budget limits.
That said GDD is a document that is tailored to any specific project’s needs, and they may be a lot different compared to each other. Some games have several pages of design documents that describe everything down to its minute detail. While others barely make it to two pages.