Hello Ckurriemi75
Yes, you need to activate a travel day in order to take advantage of a reduced Gornergrat ticket. That’s quite a waste of money as the round trip will cost you around CHF 110.
That is why I, personally, would not buy / recommend a Swiss Travel Pass Flex yet rather the Swiss Half Fare Card. The latter offers more flexibility at lower costs. The round trip would cost you around CHF 60.
Kind regards,
Roland