Your reported Functional Threshold Power (FTP) may vary slightly between software depending on the testing protocol used by each training platform, or the way you measured it outside.
Recently, a shorter "ramp test" has been the favoured method of FTP testing, rather than the traditional (some arguably harder and more accurate 20-minute test). Some riders may find the shorter, sharper ramp test gives them a higher FTP result than the longer 20-minute effort. The Kinglake FTP test in FulGaz uses this 20-minute all-out effort to calculate your FTP.
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 any test you've performed outdoors as the load on your legs is very similar.
If you plan on using one 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 your FTP can always be adjusted "on the fly" during a workout if you're finding the intervals too easy or hard.