I moved the parallel discussion to the separate topic Should you comment and/or document code?, so I will just copy over my answer about the GDD here, as it was still on-topic 
As for the GDD, I tend to have a Notion page describing the general idea of the game, along with the TODO list. Although the TODO list I’m trying to incorporate into the Godot project directly, I’m yet to find a really good Plugin that satisfy all my needs in that regard. I’m thinking about doing the same with GDD to make it a part of the project directly and be reflected in the git commits. GDD is a living document for me, so it makes sense to bind it to the project.