Rouvy user with a Core here, too.

If you ride the route in Training mode rather than TT, in settings you can adjust the Reality level up to 150%. Theoretically, the Core would then simulate up to a 24% gradient by applying more resistance. I don't know if you can vary the reality level during a ride, but I think you can. If so, when the gradient is <=16%, set it back to 100%.