Going all the way up to st.moritz is covered by the GA.

Here is a map for your GA with explanations , where you can travel for free and where you get 50% discount (and pay the other 50 %)

http://www.sbb.ch/content/dam/sbb/de...ichtskarte.pdf

If the line (any colour) is dotted(for example from Wengen to Jungfraujoch) you have to pay 50% of the normal fare(your GA is not valid on this route) plus have
the GA (to go to Wengen).