FrancesGD In such situations, it is better to buy a ticket from the last station where the train stops inside the validity area of the pass, this is Gstaad for the Golden Pass Express.
I suggest to buy a ticket between Gstaad and Montreux to avoid any problem. I don’t think that it makes a big difference.
You have reserved seats between Interlaken and Montreux. No check is made if the train is sold out when you buy only a ticket, reservations are recommended, but not mandatory. A lot of people travel with day passes or travelcards valid in every train in Switzerland, it is impossible to know in advance how many people will travel on this train.