You can set up a background with your GIF file and set the body this way:
body{
background-image:url('http://www.example.com/yourfile.gif');
background-position: center;
background-size: cover;
}
Change background image URL with your GIF. With background-position: center
you can put the image to the center and with background-size: cover
you set the picture to fit all the screen. You can also set background-size: contain
if you want to fit the picture at 100% of the screen but without leaving any part of the picture without showing.
Here's more info about the property:
http://www.w3schools.com/cssref/css3_pr_background-size.asp
Hope it helps :)