Hello Surak,
I just calibrated before trying the app so I don't believe there's an issue with the calibration of my HT, my weight and height are both correctly definded.
It's not because you post a message for the first time on a forum that you are newbie on your bike, in the real life, and I don't think that all the people that are facing the same experience are necesseraly newbies. I am sure there are experienced people who felt the same problem while ridding their first climb.
Now, I have been through the different post and I noticed that someone said "Are you using Zwift at default trainer difficulty or 100%? Default is 50% so you're only doing half the gradient. Very few people can actually do alpe de zwift at 100% trainer difficulty".
I am very suprised to here that as well, so there might be something to look at on Zwift, never heard about this setting by the way and how to change that. I would be more than happy to test Zwift with 100% difficulty to gauge the difference between both app.
But honestly, the first climb on Zwift, I was on the lowest / easiest gear of my bike and could barely turn the pedals... There's a pass close to my place, the hardest part is at 14%, I felt the same at 7% in Rouvy.