Matching %grade on a workout using classic trainer vs intended power

Hi, I am new to zwift so just figuring this all out. I have a Kinetic road trainer and speed Garmin sensors . When I’m doing my own ride or joining a non workout event I try to match what I think the resistance is based on the %grade indicated. However when doing a planned workout mode what do I follow first , the intended power output or the %grade.for instance this morning it was 3 mins intervals at 55rpm and x power. But there is no way I can make a -2% grade into 55 Rpms and the suggested power. More the rpm than the power obviously. So do I sort of just ignore the % stuff when it comes to workouts and match the intended power and cadence?
Just to make sure I am using the workout as its intended to get the most out of it. I’ve sort of been making it all up as to what challenges me and loving it ! But the workout thing confuses me when it shows the grade .

Workouts are power over time. Make sure you choose a flat route when using workout more, but ignore the gradient changes since that is not what the workout is about.

Thanks that is what I thought. As for choosing a route, this was a group workout so I assume on those the routes are not choosable. Ok I will match the power and cadence vs the grade. Thanks.