My problem so far hasn't been posting gifs, but instead seeing them.
Maybe my settings are off but all the post icons are VERY small before i clock on them, and gifs require me to click the dots and go fullscreen or they just play in the tiny preview window.
I saw an article about this on the lemmy.ml instance. The article said that GIFs are not mobile compatible and Lemmy converts them because of this. If I see it, I'll comment it here.
I checked the post it's working for me. I'm using web ui. Could the problem be with your client?