Are you asking why a GIF is used, or why the 43 bytes are needed?
I think a GIF is likely the smallest wide-supported resources format (force the browser request a URL to load a specific resource). The 43 bytes is the minimum size for a valid GIF, so the browser won't log an error loading the image and go through all load events.
Is there a reason for this?
And no, a GIF is a GIF, nothing else is a GIF, an mp4 is not a gif, webm is not a gif.