How can an app access Android Pay data on a rooted phone?
By default, only the application itself (and, under some circumstances, the applications written by the same company) can access to its (private) data. So Google Pay can store securely banking-related information.

If you root your phone, however, any application with root access can access to any folder on your file system. Including applications private data. And Google Pay's banking-related information.

I have no idea what Google Pay stores. But there is at least a way to authenticate as yourself. From there, an application with root access could probably do nasty things. Not nasty for Google, but for you: make payments on your behalf.

If you find a way to hide the root from Google Pay (I don't know if it's achievable, but I wouldn't be surprised if it was), and are entirely trusting the applications you give root access to, sure, go ahead. Personally, I would not.