how to loop gif with Sprite

function Sky() {
  tSky = new Sprite(scene, "./bg_sky.gif", 800, 700)
  tSky.setSpeed(0)
  tSky.setPosition(400, 300);

  return tSky
}

I create game but I can't animate this GIF in scene

when I set speed it will slide the scene and the scene will be gray instead.