A fix was deployed this morning, I've tested a few GIFs and was able to upload successfully, can you try re-uploading the GIF that was causing issues for you?
EDIT: A developer clarified with me that we made some changes to speed up uploading animated GIFs, there are still a few underlying issues that still need to be addressed. Will post another follow-up when ready. Thank you for your patience.
Update from Slate, 17th June -- We've completed the extent of what we have planned here. We've created an explicit limit on image size in megapixels to prevent processing delays while uploading gifs, and failed uploads should now display a more helpful error message.