Which versions of the Apple Watch is Pokemon Go compatible with?

It seems that the game is compatible with every series of the Apple Watch. Looking at some commentaries on this Reddit thread, it looks like Series 1 users are able to play it too:

Q : It's only for Apple Watch 2 or the old watch can get it too?

A : All watches I presume. Works on my Series 1

And for original series:

Q : I'm hearing it is only for Apple Watch series 2 because of the built in gps can anyone confirm?

A : Works on the original Apple Watch. I have the first

A : Can confirm! You can even set the game as a complication on your watch face.