I understand @riking’s concern, but in this case it is totally safe to whitelist autoplay+loop for oneboxed Gifly GIF-videos as they do not have sound.
Note that Imgur also does not support sound, but as a precaution oneboxed GIF-videos are muted by default:
<video id="video" autoplay="autoplay" muted="muted" preload="auto" loop="loop" poster="//i.imgur.com/YfPjduYh.jpg">
(...)
</video>
We should simply do the same for Gfycat.