Hello!
I've been creating games for over 10 years now and have worked in many different genres. I've both served as lead designer, and worked under designers as a developer and understand what is needed in a GDD.
In designing a game, I feel rapid prototype development is important as what sometimes looks good on paper does not always translate into a fun game. It's also important to stay in-bounds of a projects scope accounting for features that may add too much development time for their worth in gameplay.
As a designer, I would take the concept and prototype out any novel gameplay inclusions then supply the prototype along with the GDD to be given to the developer.