The Interlaken - Montreux Railway is included in your BO pass only as far as Saanen. Beyond that you need a ticket, which you can buy at a 50% discount. Easiest way to get that is to just go to a train station when you are here.
The reservation is separate from the ticket. The BO pass does not cover reservation charges, so that you pay regardless of what ticket or pass you use on the train. Reservations are not compulsory, however in summer they are probably a good idea. And yes, they cost 20,- pp
If I was only going as far as Saanen I would not bother with the GPX though. Just take any other train on the route.