The problem is that the platform currently doesn't support animated images or videos embedded in text lessons. So they're only available in the offline version for now.
It's a known limitation and it's on the platform developers' roadmap. But that's why we replace them with static images, and why the videos in question are mostly just short demos of the effect in action.
Hum, ok! I though that maybe since gfycat host the video or gif itself elsewhere it would be able to circumvent this problem, thanks for the answer!