Hello Kauquanuez72
There is a map showing all routes where the GA is valid. Beware that the validity differs slightly for different kinds of GA’s, so be sure to select the right kind in the top-left.
I am a bit puzzeled because I until just now thought that the GoldenpassExpress requires a seat reservation, but on the website it only states that they are recommended (except for the prestige class where they are mandatory). Apparently the reservation is not needed but still recommended.). Reservations are 20.- for both 1st and 2nd class and 49.- for prestige class (which also requires a 1st class ticket).
In any case, you can travel with the GA with no additional costs on the same line by using local trains (which means that you have to transfer in Zweisimmen and in Spiez)