If you have a Swiss Travel Pass you can treat it as if you have the GA. You will not have the little plastic “Swiss Pass” card as described in the previous answer. You have the Swiss Travel Pass for tourists, not the GA. But you can pretend you do.
So you can go on www.sbb.ch and just book a trip, and enter “GA” as reduction.
However this is not always the best way. My advise: Look up first which trains you want to take. Then check for the route you intend to travel what the border station is. In the case of travel from Milano to Interlaken that is Domodossola. Now just book a ticket Milano - Domodossola with Trenitalia. Book an EC service, to have the fast trains. Then just stay on the train in Domodossola, and show your Swiss Travel Pass when the conductor comes.