Don’t try to write a GDD. Write a series of DD for the game, and then compile them into the GDD. Character DD, Story DD, World DD, etc, etc, etc. For some people, the GDD looms too large. It can be a big project, they can feel anxiety in writing it, and it’s not long before the focus becomes the GDD rather than the game itself.
Yes, working on the parts individually can mean that there will be conflict as you go to compile/complete the GDD itself. Personally though, I’d rather have that conflict as I am putting the GDD together later than have it stall me during a linear process. Likewise, if I’m unsure about an aspect of a particular DD - I’ll put it aside and come back to it later. I’ll work on another DD. Again, it’s about not wanting to find myself stalled on some linear process.
If you have a conflict between the Character, the Controls, and the World as you’re compiling the GDD - you’ve still got the rest of the documents; it’s not a case that you reached that point and stalled…never reaching the other info or reaching it in a diminished form because of stalling. It can be tiring, even boring, to work on a GDD. You do not want that to sap your strength or resolve.
I also find that having those pieces and putting them together, working as if you were doing a jigsaw puzzle, helps you recognize what’s missing better than if you try to do it all from the start. One of those, oh snap - how’d I forget that? Well, it just did not seem as important when you started out. I believe you’d rather find things are missing while duct taping the GDD together than after you’ve started development.
Because, keep in mind - it’s just you. You’re not going to lunch with the guys and bouncing ideas around. There’s nobody else around to have an epiphany - it’s just you. There’s nobody else to slap there forehead at how stupid something is…it’s just you.
The process for putting together a GDD for an individual cannot be the same as putting together one for a team…because the process simply is not the same. If you’re trying to write it as if there is a group, a lead that has greater say, feedback from different technology teams, and all the other folks commonly involved in a project…and it’s just you - well, I wish you luck. I prefer taking an approach that’s more customized to my being a team of one and knowing how I work. It’s similar to any project that I’m working on - whether I’m writing fiction, throwing together an audio track, or developing software.