Is multimedial

A GDD is not just text. A picture is worth a thousand words, including sketches, concept art, reference pictures, or even examples from other video games (which could easily be in video format). These are great ways to convey information about how a specific system should work. In particular, using existing video games as a reference is perfectly fine.

It is unambiguous and can save both the designer and the reader a lot of time. There's no need to describe something that already exists in a lengthy manner. It's called not reinventing the wheel. If someone already did what you're trying to do, and you can reuse what's already been done…just do it!

As we already said, you don't have to be innovative in every aspect. On the contrary, using familiar mechanics where needed is as important as creating new breakthrough features. In the case of new mechanics, the GDD can contain a link to a playable prototype or footage of prototyped gameplay. Remember that as a designer you should always clearly describe what you’re showing in any media you are presenting. Showing a video of a particular game's combat system and just stating "Our game combat system works in the same way as the one shown in the video" is clearly a very bad way of writing documentation.

On the other hand, if you're working on a turn-based RPG that uses an initiative bar to show the player in which order the characters attack, you can definitely use a video or an image from an existing game: