You can watch it on whatever you want to.
But you have to sign up for it through the HBO Now APP, through Android, iOS, Amazon, etc...
It sets up the free trial and subscription based on one of those accounts "Google Play, iTunes, Amazon, etc..."
But after you sign up, you can open up https://www.hbonow.com/ on any web browser device you want and you just log in using the email/pass you created for the service. (Or just use the app on your phones/tablets/whatever)
Except for PS4 because of a "console exclusive" contract Microsoft apparently signed with HBO.
They aim for being simultaneous w/ the TV broadcast but really it is a few minutes early on Now/Go. For GoT, that is 9 eastern
Thanks!