Well, the GP route goes right through Gstaad, so going there the next day is wasting a day back-tracking. If you want to see Gstaad, you can easily get off the train and put your bags in a luggage locker while you're there.
You don't need reservations for the GP, but since you're going in high season, it might be wise to make them. You can easily make reservations just one or two days prior to your trip, from any train station in Switzerland. Reservations cost around 5 chf each, and they are separate from the train tickets (which you will cover with your Flexi Pass).
If you decide to stop in Gstaad for a few hours, you will need to make reservations for the trip to Gstaad and for the trip from Gstaad to Montreux.