My KICKR (or other trainer) speed is different to the speed in FulGaz

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.

See also: Why doesn't my Garmin / Wahoo speed match FulGaz?

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 its "downhill drive" simulation.