I spent a long time planning a game on my free time, polishing the whole concept, storyline, dialogues and events... until that day I actually sat down to start, and realised I would need either 20 years or 20 people to actually achieve it within the scopes I envisioned.
To me, GDD exist to:
- keep track of storyline / maps / flowcharts / skill trees and other complicated things in one place, if you really really need them in your game
- explain your vision to other team members, knowing they won't read it and will prefer to ask you directly when they need something
- plan changes and do a fast check of their consequences on your previous plans
So, if you are just learning the tool , forget thee GDD, make a pong.