Steps
- Right click to upload background on g-meet and click inspect (open dev tools)
- Search
<input type="file" jsname="tif8Pe" jsaction="change:E7zRc" accept="image/jpeg, image/png" style="display: none;">
Where it says accept="image/jpeg, image/png"
and replace to this
accept="image/jpeg, image/png, image/gif, video/mp4"
- Voila, Finish and try to upload your gif or mp4 file