<script>
  // ...

  function animate(node) {
    let resolve, reject;

    gsap.to(node, {
      // initAnimation
    })

    return {
      update: (visible) => {
        if (visible && reject) reject();
      },
      destroy: () => new Promise((res, rej) => {
        resolve = res;
        reject = rej;
        gsap.to(node, {
          // destroyAnimation,
          onComplete: resolve
        });
      });
    }
  }
</script>

{#if visible}
  <div use:animate>content</div>
{/if}

timeline.to(node, {
  duration: 4,
  opacity: 1
});