The short answer is - your speed shown in FulGaz should be the "trusted" and correct speed for your effort indoors on the course you are riding.
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.
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.