I am on the side that all of the parts of the DDP are equally important. Everyone needs to know the scope of the project like the back of their hand, as well as what their duties are as their respective department members. Communication is crucial as you cannot work as a team if you are not constructively communicating with each other. As for the Gantt chart, even if you ignore deadlines, it is just as important and each team member can visualized how their respective portions of the project intertwine with other members responsibilities.