There are two modes that you can ride your workout in; SIM and ERG mode.
This is where your trainer resistance is controlled automatically by your smart trainer. There is no need to change gears when using this mode. Simply select a gear you are comfortable with (big or small chainring, and a gear in the middle of the cassette is a good choice) and then start your workout. All you need to do in concentrate on pedalling and the resistance will be adjusted to keep you at the power level prescribed by the interval.
This is where the resistance is dictated by the virtual terrain, just like outside or during a "regular" non-workout ride in FulGaz. You will need to change gears to keep yourself at the prescribed power level. This is arguably a "better" mode to use, as it teaches you to vary your power with the terrain, rather than letting the trainer do all the adjustments for you.
Switching between modes
You can switch between SIM and ERG mode during a workout by tapping the "cog" icon in the top right corner of the screen, then tapping the SIM/ERG toggle.