I think the answer depends on many factors. Here are some:

#1 Do you work alone ? One of the reasons GDDs are useful is so every person in a team knows exactly what it should be working on,so it doesn't end up making a different game.

#2 Do you have a good memory ? Well if the amount of information about the game is such that you can memorize everything,perhaps you might not need a GDD. At least it doesn't have to be so full of information as some templates seem to suggest. But because game development is something that can take much time,and because you might just forget something at some point,it would be a good idea to have a record with all your thoughts about the game written.

What I do is having a folder for each project,where there are usually many text files in it. Not all files are written at the same time. Usually there is only one file written before actual development starts,and that is usually only a list of features that together detail the concept of the game,its main experience,the 'core' features. What is the basic idea,the vision behind the game.After actual work has started more and more files are added gradually.Some files are dedicated in to explaining with details how each feature will work. Other files have to do with story writing. The folder ends up with files for time planning,technology,notes of various sizes and qualities of in-game world objects,etc.

In the end someone could take all the info of the individual files and concentrate them in one file,and with the appropriate formatting,make a typical GDD. But I find that opening wordpad and dirty-writing down notes individually is a more ideal workflow for me than opening a more advanced text editor and keeping all the info in a single file.