> ...gravity pushing that food down isn't really the main driving force. What does most of that job is layers of smooth muscle, that contracts and pushes things along without you even being conscious of it.

For anyone who wants to know more, this process is known as peristalsis.

Certain neurological conditions (e.g. Paralytic Ileus) can impact it and the results of this certainly prove that gravity doesn't play much of a part in the movement of food through the gut.