GDD I think is a game design document. Preferibly in .PDF with some pictures.... Lots of work to get one set up in a legible way.
I'm lazy and juts use a napkin or any stray paper laying around, but these are for me and not to be shared.
In the GDD you need to show some concept art (does not need to be great, as it is just concept art) ..
You also need the story .... not just the back story. So, the major quests, maybe some drawings of "game play"
showing your idea of what the actual game should look like. Again, not perfect as all this will be changed later.
If you have played ANY rpg game, imaging drawing the Battle Scene with the menus systems ....
If doing more of action battle (street fighter?) draw that ...
if a skill tree, explain and draw that showing how a user would interact with it (if any, some games have this hidden)
I can go on and on ... but basically, you need the entire game described as to what you want.