A functional prototype is worth a thousand GDD words.

This means that it might be better to invest your time and work on creating a prototype that implements your game idea (or feature) instead of putting a lot of work upfront by writing down a GDD.

You can validate your game idea/feature when you have something that people can play, test and give feedback.

It is much more difficult to validate an interactive experience by just reading a document, since people can interpret it in different ways.