How can you enviar GIFs on fanpop?

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

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

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