It's the same way you put in a regular image, actually; 
However, I would advise that you download the gif you want and insert it into an image URL generator thingy if you're gonna use that method. A website I would suggest is postimages.org, since the media you upload doesn't expire after a certain amount of time. After you upload it to the site, copy the link besides the "Direct Link" label thingy, and then you can paste it into the url section of the format wtvr crap I listed above into ur bio. Hope this helpz 🫀🫀
