The GDD should include everything, if it doesnt then whats he point of the document? you may as well just tell the person programming it you want a multi-million pound revenue generating FPS to compete with COD and expect them to do the rest.
The designer/s is/are the main person/people behind the game, it's their ideas that make the game come to life, they're designing it for a reason and the more info they put into it the better the game should be.
Artwork isnt too essential though in my opinion, with a few exceptions, the GUI and main layout of the game could maybe do with a few scetches to show the positioning of options and basic structure, but characters/graphics/animations etc... should be described in enough detail in the GDD to allow for the graphics designers and stuff to come up with the designs theirselves, they're the ones trained to do so.
The designer should be able to describe anything within reason within the GDD, e.g. "Character 1: 6 ft tall, slim build female, big bust, cheerleader outfit, shoulder length brown hair in pigtails, carries sawn off shotgun with pink ribbon hanging from the handle."
And the GDD should definately change on regular basis imo, its the only way something will improve in design and principle, with the exception of the game maybe being in its final stages of development, then changes should be limited.