Kindly note, the Golden Pass train between Montreux - Zweisimmen - (Interlaken) is operated by Montreux Oberland Bahn, short MOB.
For the Golden Pass Panoramic a seat reservation is not mandatory but strongly recommended as it is very popular.
Through the following website you can book your seats. If you hold a Swiss Travel Pass, a Saver Day Pass, a Berner Oberland Pass or equivalent ticket, all you have to do is reserving the seats, the beforementioned tickets are valid:
You cannot choose the side of travel direction, the seats are automatically allocated. As the train turns multiple times during its journey, for example while it winds up through the wineyards, both, left and right hand side do offer great views.
The Golden Pass Express is again back on track, for the time being with 1 direct connection per day, see the communication here:
There is no news yet for how long the restrictions on the line will remain in place. As I said in my introduction message, for further information you would have to contact MOB, being the operator of the line:
Enjoy the ride, it is amazing.