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 π«π«
and my mom is always yelling at me to help but when I come home I'm always tired and one time I got sick bc my tooth was hurting so i threw up and was buring up well she picked me up and she said "u couldnt have waited to u got home?!" and she has been knowing i have bad teeth problems in the back and she wont get them pulled out and y dad is trying to get dental insurance for me but my mom wont even try so everyday just eating gum or food my teeth hurt or just sum cold drinks hurts so she doesnt even try to care and she gets money for me every month but still
