As a rule, GOG Galaxy is designed and maintained as a desktop application, and we do not tweak it to work on tablets, specifically.
That being said, it should work on Windows-based tablets.
There is a known glitch that - if you aren't using a dedicated keyboard - the on-screen touch keyboard may not always display when you click a text field (like the email/password fields). Switching to another app, and then back into GOG Galaxy, should help in most cases.
We cannot guarantee GOG Galaxy's functionality on other platforms, like Windows Phone.