From what I found after a quick search ("gnome animated background"),
apparently, it's a video-like effect, but the implementation comprises
several static images (e.g., a general background and several smaller
parts) and a file defining transitions (e.g., of those parts over that
background).

The person proposing this Change should supply some video showcasing this, or a very detailed description, otherwise people will have very varying ideas of how this works and looks.