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Bedlam Games, located in Toronto, Canada, and makers of the Dungeons & Dragons games, work routinely with Game Design Documents. The following is an excerpt from their GDD for the game Red Harvest. The excerpt was prepared by their creative director, Zando Chan.
Notice that the GDD contains references to audio, scripted dialogue, lore, level design (map and metrics, which are the dimensions of the space), concept art for the characters, description of the actions, and information about the interfaces including the importance of a timer.
Keep in mind that what you are reading is part of an actual GDD that many different artists, programmers, animators, etc. worked from to build the game. As you read, imagine what role you might adopt for such a project. Pay special attention to the information provided for whatever role you might take on, and determine then, how you have complete instructions on what to create, but plenty of room to develop your part of the project based on the skills and experience you would bring to the team.