Hello Presly
The answer @Danhiel has given is totally correct.
For completeness (and other readers), you are welcome to choose the GA Travel Card discount option when travelling with the Swiss Travel Pass.
However, this option must not be chosen when travelling with a day pass, for example the Saver Day Pass. In this case, a separate ticket has to be purchased for Milan to Domossola. A through ticket is not possible, as day passes cannot be combined with international offers, this is the sole exclusivity for GA / AG and Swiss Travel Pass holders.
Kind regards,
Roland