If you don't convert GIFs on server side it would take much more data than videos. I mean much, much more. GIFs are more or less a long seqeunce of 8-bit images.
Most of the "GIFs" you encounter on social media these days are actually autoplay videos without sound. So it's not that crazy. I think they're waiting until they have a robust solution figured out for video support before they land "GIF" support.
One of the alternative clients (Graysky) supports GIFs through some integration tenor.com, I believe.
Maybe they don't see reaction gifs a useful feature of a social media?
You can post GIFs but animated GIFs don’t work. They get converted at upload.