I found a solution:
- Load gif to Tenor.com or Giphy.com
- Сopy image / Copy link —> paste image to your game page
- Profit.
My solution is to put them in a public GitHub repo and link the raw content URL.
Here’s an example:

This works for GIFs in project page and devlog bodies, but it doesn’t work for banners or background images, since those have to be uploaded via the theme editor.