Hello npayne
The Half Fare Card gives you the better discount for the Jungfraujoch. For the other trips, you could buy half fare Supersaver Tickets (valid only in the selected trains) or half fare Saver Day Passes (valid in every train). You should book the discounted tickets as soon as possible, but they are not refundable and not exchangeable. The system don’t check if you already have a valid Half Fare Card or not when you buy a ticket.
For the Golden Pass Express a reservation is recommended, I would buy the tickets on sbb.ch and only the reservation here:
https://www.gpx.swiss/en/
Other trains on this route don’t require a reservation. When you search a timetable, include “Gstaad” as via to travel on the Golden Pass line.
Kind Regards
Danhiel