Trying to use .gif's in Signatures should give you this response:
I don't know more about it myself, but it seems that it's not allowed.
Btw plz try to use the Help Forum next time:
https://osu.ppy.sh/forum/5The Development Forum is made for Users to present their own Projects.
Edit: I found something in the Rules regarding Animations, it seems to be allowed but i don't know which size you will need, to make the gif allowed
I tryd a 50x50 4kbs gif but that was still not allowed.