In a Game Design Document (GDD), genre, theme, and audience are key parts that shape the game. The genre indicates what type of game it is, like action or adventure, guiding how it plays. The theme expresses the game's mood or story, such as fantasy or horror, influencing the art and narrative. Knowing the target audience helps developers create content that appeals to the players, ensuring the game connects with those who will play it. Together, these elements provide a clear direction for the game’s development.