Apr 6, 2015 - 10:48 AM
You'd better not choose the GA option as it offers a discount in Germany, but the Swiss Travel Pass Flex does not. The Swiss Travel Pass Flex offers free traveling in Switzerland, so you don't need a ticket for the Swiss leg of the trip. You just need it for the German leg, which I suppose would be Basel - Berlin.
The simplest way would have been to order it along with your pass. You already have the pass, but you can still order it separately. Since you live in Hong Kong, you can get a ticket from Interlaken to Berlin for Swiss Travel Pass holders through www.raileurope.hk. It includes a seat reservation for the whole trip and a discount because the Swiss leg is free. Or, you can just book the German leg on the SBB site (do not choose GA), or through the German railways at www.bahn.de.
I hope this was useful for you!