The way to get the best out of FulGaz when using a "classic" home trainer (dumb trainer?) or spin bike that doesn't have variable resistance is to change up to a harder gear when the road goes upwards, then down to an easier gear on descents.
This is the exact opposite of what you would do outdoors. It only takes a couple of rides for this to become surprisingly instinctive because we're used to changing gear when we see a hill in front of us anyway.
As for "what wattage should I ride?" That's entirely up to you in FulGaz rides, just like it would be outdoors. The exception to this is if you're doing one of the FulGaz training program rides. These have recommended wattages throughout. you then aim to stick to those power outputs using the help you see on the screen.