Turn animated gif into WhatsApp sendable video
You can use this online tool to convert it: http://www.giftovideo.com/
As a side note regarding the online tool, at this exactly time, they seem to be facing some performance problem, for every time you try it, you're going to end up in a queue like forever.
If you're happy with a solution that you can use on a PC, then SuperUser.com has this: How do I convert an animated GIF to a YouTube friendly video format?
For an on-Android solution, none of the video coverters specifically mention this but as the ffmpeg library does support this (as seen in the SuperUser answer), apps like Video Converter Android or Media Converter and so on, being ffmpeg based, should be able to do it.