You do not need to add the pass to the SBB app to get discounts. You can get discounted tickets even without. Wether you have a particular reduction card is not checked during purchase.
But for trips on cable cars etc. just go to the ticket office. You should not book these in advance anyway.
For. Zurich - Venice you are best off to just buy a ticket Chiasso - Venice on Trenitalia. Then just board the train you booked in Zurich. The alternative would be to mention “GA Travelcard” as reduction when booking on SBB, but that will probably end up being more expensive.