The short answer is - FulGaz is the correct speed for your effort indoors on the course you are riding! Your turbo trainer does not know if you're going downhill or uphill or hammering along on the flat.
To put it simply, the two will never perfectly match – and that’s not a problem!
For the first 3 seconds, FulGaz starts at the original rider’s speed before switching calculations based on physics for a rider of your size, weight and power production. It is your power output that determines your speed in FulGaz, not the speed on your turbo trainer.
EG: If you stop pedalling on a steep descent in FulGaz, your data on-screen will continue to pick up speed (just like you would in real life) but your trainer would slowly grind to a halt. One notable exception to this is the Tacx NEO with it's "downhill drive" simulation.