Rail Ninja is a scam site that preys on people that are unfamiliar with the European rail system. They will happily take your money, but will not give you a ticket either.
Trains are not complicated. They are mass transit. If you were to go to the Zermatt station right now and ask to buy a ticket to Montreux you would get one. Then you would just get on a train and travel.
Buying in advance does give you an opportunity to get. a discount, but for that it is not necessary to buy half a year ahead of time. You can safely wait.
Final note: If you have a “Swiss GA PAss” (I assume you mean Swiss Travel Pass) you do not need a ticket at all, as your pass is your ticket.