Hi Sougauji38
Reservations for the Golden Pass Express are never visible in the SBB Mobile app. They are not linked to the SBB system.
You can buy a Point-to-Point Class Upgrade only for this trip on sbb.ch, in the SBB Mobile app or at a ticket office. Choose first the timetable, select the train and click “Purchase ticket” (see example). I suggest to create an account and not to buy tickets as guest.

Kind Regards
Danhiel