Short answer "FulGaz is the correct speed for your effort indoors"
For the first 3 seconds FulGaz starts at the original rider’s speed. After that FulGaz obeys the laws of physics for a rider of your size and weight and the power you are producing. It’s this power reading that determines your speed in FulGaz, not your speed on your trainer. This means if you stop pedalling on a steep climb FulGaz will grind to a halt more or less immediately, but if you stop pedalling on a steep descent FulGaz will continue to pick up speed, just like you would in real life.
This means it is only possible for your trainer speed and FulGaz speed to match while you are going up hill. Even then, if you've adjusted Slope Scaling this will massively effect the speed of your trainer relative to the speed in FulGaz. It's all nothing to be concerned about.
Before you look into this issue too deeply, make sure you’ve done a spindown first. This can make a massive difference to how the KICKR feels, reacts and reports power. You need to do this in the Wahoo fitness app.
At the end of the day, we can make all the recommendations we want, but a bit of personal tweaking may be required. The way to get really precise is to weigh yourself (holding up your bike wearing your bike clothing), then film a ride. We can then take your outdoor numbers and compare them to your FulGaz numbers, then tweak settings accordingly. we’ve done this for some pro riders, and we’re happy to do it for you too. (It also helps if you visit a wind tunnel, but that’s probably taking things a bit too far).
IMPORTANT NOTE ON SPINDOWNS AND CALIBRATION
Once you have done a spindown, it's really important to then quit that app before you use FulGaz. If you don't do this, your trainer will still be paired to that app and FulGaz will not work. In some cases you will also need to "forget" the trainer in the utility app.