Your functional threshold power number will always vary slightly depending on the testing protocol used by each training platform (or the way you measured it outside). Some riders may find a short, sharp ramp test gives them a higher FTP result than a longer 20-minute effort. It's a little personal.
Your FTP will also vary from day-to-day depending on how fresh or fatigued you are. While your FTP is important for setting interval targets in FulGaz workouts, it's best not to get too caught up in the number or compare it to others.
Your FTP in FulGaz is likely to be closer to to any test you've performed outdoors as the load on your legs is very similar. If you plan on using on of our training programs, we would recommend completing the Kinglake FTP test in FulGaz and using the result for that. Our training plans were designed around the result this classic style 20-minute effort test gives.
Ultimately though, the choice is yours and it can always be adjusted on the fly during a workout.