My avatar is evidence enough that gifs can be used.
Here's the thing about it. If the gif in itself is bigger than the dimensions allowed by your ranking (which in your case is 120x120 I believe.) then PC will automatically resize and reformat it into a static image. That being said, you can use this to resize your gif.
http://ezgif.com/resize
Steps to resize image:
1. Upload file/copy and paste image url of gif
2. Resize image if image is too large. Note: if you're going to reduce the size of the image, make sure you do it by the percentage of the gif. What I mean is if you have a 500x250 gif and you want to resize it to a smaller size, do it by like 20% which would make the new dimensions 400x200 (100 is 1/5 of 500 and 50 is 1/5 of 250.) By reducing it by a percentage instead of just random amounts, you are keeping the pixels intact within the picture.
3. If you just want a specific part of the gif after you resize it, click "crop" in the hud bar. This will allow you to take just pieces of a gif and make it the desired size (note: you don't have to do the percentages in that part.
4. Once you're done hit "save" in the hud bar which should download the image, then you can upload directly from PC to make the image your avatar. If everything is done correctly the gif should work.
Hope this helps, mate. If you have any questions feel free to shoot me a VM and I can work you through the steps.