Animated GIF emojis are available on many android keyboards, e.g. Gboard from Google.
Not sure about "in theory", but sure you can reproduce it in comparison to Gmail:
In Gmail app, while composing new message, pick any Gif emoji from Gboard, and send it to any email address you have control on. Check both Sent and the receiver inbox, and you will see the animated gif in the email body.
If you try to do it in AM, you will get that error message.
Not a big deal, BTW..)