You don’t need a power meter. If you have a smart trainer, then the trainer can report power to the app. If not, then the app has power/speed curves for a lot of non-smart trainers, so you can get a predicted power numbers, as long as you have a speed sensor that can connect to the app.
If none of the power options above work, you can do the workouts using RPE. That’s how I started out in my early days of doing SUF videos.
The sessions are all designed for doing indoors on a trainer. Some of the training plans have outdoor rides in the schedule. At the moment, there is not an option to send one of the workouts to your bike computer for doing outside, but I think I’ve heard that may be coming in the future.
There are a number of training plans that target Gran Fondos, so you could have a look at those. They should be good preparation for your target events.
Feel free to ask any other questions you might have.