Getting the cadence correct in these sessions is super-important.
If this is proving to be a problem for you, the good news is you don't need to fit lower gears on your bike. You can make this adjustment in FulGaz.
Go to Settings > Advanced Trainer Settings and Adjust slope scaling down to 80 or 90%. This isn't cheating, it's just a way to allow you to pedal at the prescribed wattage and cadence on even the steepest slopes. It will not effect your speed in FulGaz.