How can you submit GIFs on fanpop?

That's how gif. Files work
When another user downloads it, it can move in their desktop browsers, or phone gallery in my case

Most sites are set up that way. Otherwise, you'd be spending an hour to load one awnser on a question

If you are trying to use a gif as an icon or in an answer it won't work. However if you upload it to the images section, it usually works.