You can't actually send them unless the apps detect the remote user has an Apple Watch, they lock down into the basic set.
I know they work on iMessage, but anyone tried this? Are they GIFS?
They're .emoji files, I've not looked in to the format yet. You can see them in your sent messages on your phone.
That's with digital touch but I have sent messages to people without apple watches like this.
Yes, they show up as animated but with a black background, which is slightly inelegant considering the messaging background is white.
Have you seen how they show up on Android?
Yes, it does show up like we see it on iPhone with black background.